The Symbol that Looks Like an F?

What is the symbol that looks like a capital f, F, which is used to prove the martingale:

• It's a capital $F$ in a calligraphic font. – Matthew Leingang Dec 13 '16 at 12:39
• Without any context this is just a guess, but it might be a $\sigma$-field? – dinosaur Dec 13 '16 at 12:43
• @dinosaur It was used to prove martingale – Betty Dec 13 '16 at 12:49
• In general - filtration. In this particular - hard to say. – EzWin Dec 13 '16 at 12:53
• $\mathcal{F}$ to produce $\mathcal{F}$ for a filtration – Henry Dec 13 '16 at 15:18

My best guess is that $\mathcal{F}_s^B$ is a Brownian Filtration, with respective time $s$. See for instance Brownian Motion and Stochastic Calculus- section 2.7, from Karatzas and Shreve.