Hi,
Assign activity may be considered as a subset of Invoke Method.
Assign activity can only be used with expressions or methods that have a return value. Instead, it is very readable than Invoke Method.
Invoke Method, on the other hand, can be used even no return value. It also supports ref and out arguments and get its value, that are not supported in Assign activity.
Therefore, expressions or methods that have a return value usually use an Assign activity for readability. And others (for example, have ref or out arguments and want to get its value, and/or have no return value) is used Invoke Method.
Regards,