[admin note: this thread was edited to focus on the purpose of Bentley Communities... and that is Bentley products.]
Bentley BD uses specialist Forms, straight from original 1993 BricsWork, which Bentley bought in 1995. BD can also, as a workaround, make use of standard Bentley Microstation dgnSolids with BIM data (Parts) applied, same as BricsBIM applies Materials to dwgSolids, but it's unsatisfactory for several reasons -
While BD Forms have the vital ability to be multi-layered into Compound Parts for grouped installation and modification, that ability is lost when using Bentley dgnSolids as workaround instead of BD Forms.
While BD Forms have another vital ability, to effectively Extend To as well as Trim Back to Connect to other Forms, that ability is much degraded when using Bentley dgnSolids as workaround instead of BD Forms - dgnSolids can only be Booleaned, after being manually stretched to overlap.
While Bentley dgnSolids enjoy a very sophisticated PushPull tool, using this (and most other dgnSolid Bollean tools) on a BD Form results in its demotion to a 'dumb' dgnSolid, losing the important abilites of BD Forms mentioned above.
While BD Forms do have the ability to effectively Extend To as well as Trim Back to Connect to other Forms, that is fatally limited, only Wall end to Wall end, and Wall top to Roof underside. BD Forms fuss a lot as to which is their Top, Bottom, Ends and Sides, and refuse to Connect in most of the combinations.
Putting all of this together, the effect is that Bentley BD is fine for conventional commercial cuboid office blocks and 'decorated sheds' but struggles with any creative shapes, even quite ordinary intersecting pitched roofs.
BD has tools to create a limited range of dumb roof-shape blobs, but it very soon fails in creating novel roofs out of intersected full-ability BD Forms.
BD also has tools for creating complex slabs - but similarly crippled by inability to do so as multi-layered Compound Parts;