It is the Presumptive Mood: a) Present Progressive "vei fi ştiind" (forming: the auxiliary 'to be' in the Present Future + the gerund of the lexical verb) ; b) Perfect with a past meaning "vei fi ştiut" ( the same as above for the auxiliary + the participle of the lexical verb).
b) Eg O fi aflat vestea? - I wonder whether he has heard /could have heard the news. S-o fi gândit vreun pic la mine? - Is there any chance for him to have thought of me? Might he have thought of me a little?
Or "he might have done it" - Poate că o fi făcut asta.
a) You'll be reading the letter when I be away. - Vei fi citind scrisoarea când eu voi fi departe.