Start from the top. When planning out a season, there are a number of steps and I’ll use the list from Coach Joe Friel’s The Cyclist’s Training Bible 5th edition:
Step 1: Set goals
Step 2: Determine objectives
Step 3: Set Annual Training Volume (hours or TSS)
Step 4: Prioritize Races
Step 5: Divide Season into Periods (e.g. Base1, Base2, …)
Step 6: Estimate Weekly Volume (hours or TSS)
Using that framework to create an Annual Training Plan, the most basic volume measurement is hours. When you get to step 6, the amount of hours per week will vary based on the phase of training (e.g. base3 vs build2).
TSS improves upon hours as a planning tool, because TSS factors in both hours and intensity.
That provides a high-level plan. From that a detailed week-to-week plan is put together. If you are coached, your plan is likely more dynamic, personalized, and updated weekly or monthly. If you purchase a plan (TR subscription or from a coach), the plan is static unless you make adjustments.
So if someone else is developing your annual training plan, or you don’t have one and are just following a standard plan purchased from TR or a coach, then you can ignore TSS/CTL and just follow the plan.