Block Weaves vs Unit Weaves
Thank you for your help,
This is a good question. First of all, as I can see you know, all unit weaves are block weaves (but not all block weaves are unit weaves). Unit weaves contain threading and treadling units that have within them all the interlacement necessary to the weave structure. For that reason, you can thread and treadle many successive units of the same block or only one unit of that block (as you cannot in overshot, for example, because the pattern float does not interlace within the block). You also can weave either pattern or background in selected blocks without restriction (in overshot, for example, you can't weave pattern in all the blocks or background in all of them).
For 4-shaft Ms & Os, the restriction is that you must weave pattern in one block and background in the other or vice versa. This therefore limits the block designs (profile drafts) you can use with Ms & Os. Unit weaves by definition can be used with any profile draft without restriction.