Work in SMM is often divided into staff and project. Established - when a specialist receives a fixed salary and a non-fixed number of projects. Of the pros - stability, and of the minuses - just the fact that it is from such specialists that they are expected to be multi-armed Shivas. This option is more common in hired office work.
In project employment, specialists receive payment for the project. Many projects - big pay, few projects - you suck your paw. This is how many agencies work that hire remote employees. Here, as a rule, a person performs a clear task according to TK, which is also clearly paid. No task - no money.
There is another payment format that SMMs offer - payment for leads or sales.