Tagged blocks is sequence of varied size blocks that grows down(like stack).
Each tagged block consist of two parts - varied size payload and 8 byte ender.
last but one dword of tagged block is size of that payload. last dword some kind of type(I guess). in case of addressing_space block its payload starts with qword address folowed with 1 byte sign. tenth byte of addressing_space is virtual section marker.

I would be very pleasure to know about all kind of fasm 1 tagged blocks and their structure.
thanks in advance.

As I recall, originally these blocks were just for the buffered DISPLAY content, and I added the tags relatively late into fasm's development, when I needed to allocate memory for additional structures (like the addressing space blocks). I have the details written down in the old fasm's dev notebook, but I don't have it with me at the moment. I will return here when I do.