Thanks everyone for taking the time to respond.
This is what I gather so far.
First, the fP.
Your playing along (ppp,pp,p,mp,mf,f,ff,fff,cresc,descr) whatever, it doesn't matter and then you see the sign fP. You change your dynamic,not gradually, but instantaneously to forte for exactly one note only, and then the next note drops dynamically 3 "relative" sound levels lower to piano skipping mf and mp in the process. There is no accent whatsoever. Essentially fp could be written as two separate signs. The ordinary f, forte and then a p, piano would accomplish the same thing. So, apparently then f+p=fp. Is that all there is to it?
Now the sFz.
I am familiar with light, medium and strong accents. They have symbols and names.
I know of:
staccato and staccatissimo, the two light accents. A dot and a wedge respectively.
martelato the strong accent. A carat, capital Lambda or an upside down letter v. It looks like this ^.
marcato and tenuto, the two medium accents, > and -.
So just how strong is the "attack" of a sFz. Is it always the strongest of the attacks even stronger than martelato ^? Or does it depend on the context so that the strength then is variable from one situation to the next?
Thank you, Joe.