grammar - Which part of speech is "Be assured"?

Which of the spans in 1.1 is the stronger? Be assured, you are in numerous company if you pick the left one at first glance, where the struts and wires are doing nothing useful at all.

Could you please tell me what is "Be assured" here?

I mean whether it is an adverb, adjective, so on?

And, would you please show me some examples exactly like such the following, considering my original sentence?

Be assured, ...

Answer

This kind of constructions are mostly found in conversations. But they don't remain in the realm of conversation only. They are used without the subject as the subject is predictable and obvious. Here in your example the subject that is missing from "be assured" is "you". The full sentence will be "You be assured..."

More example sentences -

1. 
   

   Take it from me they can't come back to the game from this stage. = You take it from me that they can't come back to the game from here.

   
   


2. 
   

   Listen, be careful with that. You are dealing with something sensitive. = You listen, you be careful with that. You are dealing with something sensitive.

   
   

N.B I completely agree with MMJZ. I added this answer to add something extra to what he already told, and of course to provide some more example sentences.