how to word the requirements

I'm working on the requirements section of the use cases doc.  It seems
to me that there are three entities involved:  the user, the
application, and the browser, and that the requirements state what the
browser must enable the application to do (and what it must not allow
the application to do - for example, access the camera without
permission.)    So I have two questions.  First, do people prefer
wording a) or b)?

a)      The UA must be able to capture multiple streams
simultaneously...

b)      The UA must enable the Application to capture multiple streams
simultaneously...

 

Second question:  should we say 'browser' or 'UA'?  Is there a
difference?

 

Thanks,

Jim