Your certification covers the guidance METHOD, not one particular trade. That is why your scope belongs to you: you change it freely from your space, with no revalidation and no new exam.
What it does
It determines which missions are offered to you. A wide scope exposes you to more proposals; a narrow one brings fewer, but on trades you genuinely know.
Sectors are derived, not chosen
You select trades; the matching sectors are derived automatically. Two lists kept separately would eventually diverge, and you would receive inconsistent proposals.
Removing a trade
A trade held by an open mission cannot be removed. The refusal then covers the whole save: nothing is half applied, which stops you believing a removal took effect when it did not. Finish the mission, then try again.
Avoid Every change is logged with its date. A scope that shifts with each proposal received is visible, and reads as a lack of seriousness.
Three trades you master beat twelve you skim. A poorly guided client leaves a review, and it stays.