What does it mean when a professor says that he'll pay me as a 'mission' for an internship? I applied for and got selected for a summer internship at a renowned research lab in Germany. The professor says they will try to fund all my expenses, but will pay me as a 'mission'. What does this mean? I have never heard of this phrase before.
In my experience, there are typically three methods for dealing with funding for a short- to medium-term visit:


Directly collect travel and living expense information (e.g., via receipts) and advance/recompense money to those documented expenses.  This can be quite painful to actually implement for anything more than about a week.
Take on the person as a short-term employee, paying them a sufficient wage that it is expected all told to cover their travel and living expenses.  This can often be complicated due to subsidiary problems with HR, hiring policy, etc., and may often be much more expensive for the host depending on how their overheads are calculated.  
Give the person a lump sum of money that is expected to be large enough to cover their travel and living expenses.  This is the simplest method, but requires available money that is allowed to be used for this purpose, which is often hard to come by except in specific visiting scholar funds and the like.


My guess is that the statement about "mission" may be an unusual or mistranslated phrasing referring to the third type of funding.  The only way to know for certain, however, is to ask for clarification from the person who used the phrase.