BMIDE 2-Tier or 4-Tier is the mode in which your BMIDE Client connects to TC when required.
There are many configurations in BMIDE which require a TC session to be established; e.g., Type Display rules.
You can also have a standalone BMIDE with which you cannot establish a TC connection.
Hope this helps
NX 11.0.1.mp01 | SW 2016 | TcUA 11.2.3