You send me, with specifics.
In the world of rules and regulations, you may be sent somewhere to do a thing for money.
You have an assignment. One that is important to the superstructure that feeds you.
Your assignment is defined on paper, in an assignment letter. Because this is all formal and all.
You get your terms and conditions. Like where-to, for how long, to do what, and why, and how much you get out of it (besides being allowed to keep your job).
And when, and how, and presumably if you get to come back, which is the usual thing, though you ought to check to be sure.
And you never know who's going to remember this all, so you need it in writing. In a letter. An assignment letter.
And then make sure it's backed up somehow. Airtight. You know, just to be safe.