Kind of a vague explaination but I think I might know what you mean. I have heard them called 'tasks'. A Dom assigns his submissive a task to do while they're apart and then she either writes about it in a journal or e-mails him. Usually given so he can find out about her or to get her to try something new. Often used just as a means for the Dom to maintain control over the sub and also can be used as puinshment.

Is that what you call missions?