There are two types of visa which are referred to as Mission Visas in Dubai and the UAE.
Special Mission Entry Visa - this is a 16 day visa also known as a Dubai transit visa (but is not the same as a 96 hour transit visa). It is valid for single entry only, and is not renewable. Issued by the DNRD or other UAE NRD.
Mission Visa - this is a 90 day visa (not 3 months) for the purposes of allowing expats to work for a short time in the UAE, or for workers on probation, and is applied for at the Labour Department by the sponsoring company. It may be renewable and/or multi-entry (conflicting information supplied by official sources).
Temporary Work Permits
A Mission Visa is not the same as a Temporary Work Permit (TWP) as far as we understand. A TWP is a work permit only, not a visa, and is only issued by the Ministry of Labour (MOL) to those already holding a UAE residence visa.
Khaleej Times reported 09 August 2010 that The MoL has affirmed that a valid residency of not less than six months and availability of a job offer with a new sponsor are required for issuing a temporary employment permit.
Mission Visa application
A Mission Visa needs to be applied for at the UAE Ministry of Labour (MOL), not the immigration department / DNRD / FNRD (which is presumably why you won't find it on the DNRD website list of visas).
Mission Visa application needs to be submitted by the PRO of the sponsoring company.
Photo and passport are required, after the Temporary Labour Permit is received, the applicant then needs to do a medical test to complete the application and receive something called a Mission Card (presumably equivalent to the Labour Card).
The Mission Visa is valid 60 days before entry to the UAE, the holder can stay up to 90 days after entry, and it is renewable once for another period of 90 days.
Cost of Mission Visa is 500-600 dhs, renewing costs another 500-600 dhs (or 1200 dhs according to some sources). When visa rules changed in July 2008, reports said it cost 200 dhs and was not renewable but information received in May 2009 was that it cost 500 dhs and was renewable. Add an update to the Mission Visa Dubai topic if you can clarify. MOL website procedure for Electronic Mission Permit (EMP) says AED 100 application fee + AED 500 for approval + AED 3000 deposit. And for renewal says AED 500 + AED 500 penalty if renewal applied for more than 7 days after expiry of original permit.
Refundable depost of AED 3000 dhs is required.
There may be a restriction on job title for female applicants (engineers only?).
Transferring from Mission Visa to normal work permit and labour card is possible, not clear if employee must leave the UAE or not. Latest information (May 2009) is no, but back in July 2008, Humaid bin Deemas, Acting-General Manager of the MOL, was reported as saying "The engineer must leave the country because the mission visa was issued for three months and extended for three months. Hence, the worker must depart from the country and the company can then apply for an employment visa to bring him back and complete the formalities of either residence or employment visa," (Khaleej Times 22 July 2008).
If the applicant is already in the UAE (on a tourist or visit visa for example), they might have to leave the UAE to cancel their tourist / visit visa, and re-enter on the Mission Visa.
Other UAE Mission Visa information
Not clear if Mission Visa is multi-entry or not - MOL website says "If a worker departs from the UAE during the validity period of an EMP, he/she is barred from re-entering the country." but information later received from the MOL indicated that it was a multi-entry visa.
There is a grace period of 7 days after expiry of the Mission Visa within which the holder must exit the UAE. It would be sensible not to rely on that though. Aim to leave before Mission Visa expiry date then you have a few days in hand if a flight gets cancelled or there's another last minute hiccup.
On expiry of a Mission Visa (or when the worker exits the UAE), there is no labour ban imposed.
If a worker on a Mission Visa wants to change jobs, it should be possible to do so without needing a No Objection Letter (NOC), but the worker might need to exit the UAE and enter on a new visa.
Workers with a Mission Visa in a dispute with their employer should go to the Labour Department to file a complaint.
Anyone who has been banned from the UAE for a period of time cannot apply for a Mission Visa during the ban period.
Companies that wish to arrange Mission Visas and are registered with the Ministry of Labour can apply for Mission Visas up to a quota limit of 50% of total number of workers if less than 500 employed, and 100% of total if more than 500 employed. New companies not yet registered with the MOL need to submit a bank guarantee of AED 24,000 to the Ministry of Labour which will allow them to sponsor up to 5 Mission Visas (MOL information, last accessed 27 May 2009).
Previous Mission Visa information
Mission Visa infomormation valid prior to new visa rules introduced in July 2008. Out of date but left here for reference and comparison with other undated information.
The Mission Visa was introduced late 2007 / early 2008 and intended for people working in the UAE temporarily with the permission of the Labour Ministry. The DNRD website information is not very clear - this information comes from articles in the press.
Visa is issued for 3 months, cost is 600 dhs. Extension for another 3 months costs 1200 dhs. Maximum stay is 180 days (so if you stay 6 months, you may be in trouble for the extra couple of days although there is apparently a grace period of 7 days for 3 month visas - may not apply for the second 3 months).
Apparently you need to get a Temporary Work Permit from the Ministry of Labour first before applying for the Mission Visa. MOL website does not have further information about a TWP (at least, not in English).
According to the Interior Ministry, it is only available to expatriate doctors, engineers, lawyers and technicians.
29 September 2007 - New 6 month Mission Visa (superseded by new rules July 2008)
The Ministry of Labour is apparently planning to introduce a new visa, the Mission Visa, valid for residency and employment up to six months. Cost of the Mission Visa is expected to be 1200 dhs. This sounds similar to an already existing Mission Visa, but with no restrictions on job categories. It may be possible to change a Mission Visa to a normal Employment Visa. Information from report in Emirates Today 28 Sep 2007 about the new Mission Visa.