There is a very big difference between CTASK and STRY. Fundamentally different. Almost all work in your Development Pipeline (EPIC/STRY) should go through Change Management (CHG/CTASK) as a part of the release phase. So, in theory you need both CTASK and STRY.
The reason you don't estimate a CTASK like you do a STRY is because the Stories typically need that data (how long it will take to develop a digital product for release), whereas the change doesn't.
Change Management as a process, doesn't inherently care about how long (story points) an implementation takes...it is more concerned with what other changes are going on during the implementation window (regardless of the length or complexity).
Basically, you're trying to achieve different goals in Change Management, than you are with getting development projects approved by presenting an Epic or Story to a Business Stakeholder...Change Management ensures the implementation is successful, the "how" of your release...whereas the "if" of your release has already been decided way earlier in the Digital Product Release process.
You may want to include information regarding the complexity/effort/implementation-time-spent for the release, in your DPR lifecycle...but you wouldn't do that as a part of the development phase. Check out the DPR on-demand course in Now Learning, and that should help with understanding how all of these specific processes interact with and relate to each other.
Digital Product Release (DPR) Bootcamp On Demand - Now Learning
