Workflow vs Subagent
The plan a process follows, versus one dispatched unit that can execute a step in it
Workflow
The plan: a defined sequence of steps a process moves through in order. A workflow can be executed entirely by one agent working straight through, or can hand individual steps off to separate subagents.
Subagent
One dispatched unit of execution: a separate agent run with its own context window, given a bounded piece of work, that reports a result back. A subagent handles one step, it is not a sequence of steps itself.
Which one do you actually need
Workflow is the sequence, subagent is one way to execute a single step in that sequence in isolation. A workflow can run start to finish without ever dispatching a subagent, and a subagent can be dispatched for a one-off task with no formally defined workflow behind it at all.