I'm a bit 'confused..Sforzando is a sfz player and is also a vsti player ?

Sforzando has multiple applications. Sforzando can be used in standalone mode or as a MIDI directed instrument inside another program such as a DAW or Band-in-a-Box.

Kontakt, Samplitude and Halion are other examples of programs that have insert into a DAW track and play an instrument mode but also have a standalone mode.

My question is: can I use Sforzando for play Vsti instruments?

What is VSTi, well as I understand it, it is VST instrument the (i) indicating an instrument. It the case of Sforzando it is a VSTinstrument that plays sfz files which are files that indicate how a bunch of samples are used to make up an instrument. For, example one might have a series of .wav samples that make up the notes on a guitar (or any other instrument) an sfz file would then say how each of the samples are played to make up a scale or whatever that can be played on a keyboard or entered into a midi file using some other form. Notation or maybe a piano roll. You would load the sfz file into Sforzando and add Sforzando as the synth to play the midi part. The sfz file simply says if you hit this note on a keyboard or midi file (say C) play this sample (hopefully a .wav that is a recorded C). This is fairly simplistic sfz files can carry more detail than just notes but that is for another time.

Just the way I understand it

Tony

As Teunis stated, Sforzando isn't used to play VSTi instruments, it IS a VSTi instrument that plays samples in various formats (it will support SFZ, SF2, and some other formats).

One thing it is not is a GM multi-timbral synth, so you have to explicitly match instruments to channels when you use it.

You can read more in the Sforzando User's Guide ++ HERE ++.

Hello,
I understood many things from your explanations. Can we therefore say that Sforzando is equivalent to Kontakt as a conceptual behavior?

Yes, as well as other VSTis like UVI, SampleTanki 3 or 2.5, Play, etc. Kontakt is the kind of sample players due to the fact that there are many third parties making samples for it. However the others are very good also and they all have to be set up in BiaB or RB the same way.