How to identify the correct primary actor from the use case diagram?

Question

I have created a use case diagram based on a scenario, I identified 5 actors in total.

However, I am trying to identify the correct primary actor.

I am undecided between Team Leader and Maintenance Engineer as primary actor as they work pretty closely.

Answer 1

There are two levels at stake here.

For a whole system or a whole use case diagram there may often be several primary actors:

The primary actors are the ones for whom the system is built; they are the ones to whom the system provides sufficient economic value to warrant its construction.
- Bittner & Spence in Use Case Modeling

At the level of a single use-case, there may often be one primary actor:

The primary actor of a use case is the stakeholder that calls on the system to deliver one of its services. (...) The primary actor is oten, but not always the actor who triggers the use case.
- Cockburn in Writing effective use cases

In your case, it's even more ambiguous since you seem to have 13 use-cases split between 3 different systems (and some floating in-between): at which level are you looking for a primary actor?

Hints, not related to your question**

1. reread the narrative carefully, and make the difference between the location where the users are assigned, and the software system they are using (I suspect there's only one system at play);
2. keep in mind that the use-case is not a user task outside the system but a goal that the user want to achieve with the system (e.g. does "Return parts" correspond to an activity in the system?)
3. keep in mind that some actors may correspond to other systems that shall interact with the system you are design (e.g. is FMS really in your scope or is it a supporting actor?)