Definition of sub-system
All workflow systems have at least 1 sub-system. Where they only have 1 then the term sub-system is pointless, the term becomes useful where you add multiple sub-systems into a single workflow system.
Each sub-system contains its own stages, fields and deadlines and can have multiple workflow route variations.
On a recurring workflow system you can set for each client to be active or not active for each individual sub-system and you can also set a route for each sub-system too.
When a job is created with multiple sub-systems active then each component of the job card will be present for each of the sub-systems, you'll see a second/third/etc current stage description, progress buttons, deadlines, stages, job fields etc.