I am going to answer the question
What was it again?
OH what is Unitary...that is for me an term that WAS NEVER used but applies to FIXED ( what some of us call "bricks") PLC..like an AB SLC 500 L40. Siemens S7-200. AD DL05 etc
Modular: That is a system that is in between fixed (brick/unitary) and rack type. It uses a rack type system but CPU and housing is built in..Allen Bradley SLC 5/01, some GE Fanuc 90/30, etc.
Rack: That is a system that use uses a RACK that power supplies, CPU's, I/O and remote adapters are used in.
Each system has its use, Modular usually offers more options than unitary...rack usually offers more options than modular.Options like number of I/O count, analog, and communication etc.
BTW....I dont buy the guilt trip attempt by reversing the scenario. This was a blatant HNC question which obtained blatant answers...and by some that have done HNC. Its my belief that a skilled person investigating any subject will find answers to scholastic questions on their own...even if its just doing a search on a site like this.