BrainModular

Is it a midi buffer?

yes a kind of buffer.
you specify a buffer length and it records incoming midi events while playing old ones.
Like an endless overdub recording.
You have also a clear button.
Easy to create hypnotic melodies, in a 'steve reich' way (???)
Can be coupled to a midi delay patch (include in the patch) to make very fun stuffs.

Is it possible to quantize recording on bar or beat (start and end point sync to host)?
Can buffer lenght be automaticly adjustet while recording? Is it possible to controll start of recording with midi event such sustain pedal on and to stop recording with sustain pedal off?

the midi accumulator is designed as a 'midi special' effect.
For your needs use the pianoroll. It has a lot of quantize, start, sync, options.

Where do I find the Midi Accumulator? Is it a module?

The Midi Accumulator is one of six midi effect patches/scripts, and can be found in the Add-ons section. Look for "Midi Scripts Serie I".