The meaning of "will" here

It is a combination of command (imperative) and question.

All personnel will evacuate the building immediately!

So although the structure looks like a question, but, it's not; it's an imperative question with emphasis in fact. Agree?

Sorry I meant "imperative statement with emphasis".

The imperative would be:
Evacuate the building.

The subject (implied) is you.

But the speaker wanted to make the subject explicit (all personel). Perhaps there are some others that need to remain in the building. The only way to do this is to use the helping verb "will."

It is a form of a polite request. The emphasis can be added by adverbs (immediately, now)

@AlpheccaStars:
>> The imperative would be:
>>Evacuate the building.

>> The subject (implied) is you.

>> But the speaker wanted to make the subject explicit (all personel). Perhaps there are some others that need to remain >> in the building.

Yes, I see and agree with this section.

>> The only way to do thi

It is a question if you add a question mark.

Will you please pass the butter?
Will you all please evacuate the building?

OK, thank you.