While working with a coach recently, I was reminded of the idea of key workouts, but with a new and better term for them, "Foundational Workouts". This is the same as the key workouts sessions, in meaning, but I like this new term better. The reason is because this workout isn't just key, it is the workout you build the fitness in the weeks ahead on.
Foundational workouts are the workouts you begin your training plan for the week/month/microcycle with, and build on top of them, and around the stress they create. The idea is you should come into these sessions with the least amount of fatigue as possible, in order to maximize the quality of the session for adaptation to the stimulus.
If you're a runner, these sessions are your highest quality runs, like tempo runs and track sessions. It could be a long run, but it is likely that your long run is not the highest quality session, and the goal is simply building basic aerobic endurance, which can be required in a fatigued state, (relatively speaking).
If you're a triathlete, you might have 2 foundational workouts per week, per sport. If you're trying to focus on one sport which might be a weakness, such as cycling, that you might shift the equation to 1 swim, 3 bike, and 2 run foundational workouts per week. If you have the luxury of being a professional triathlete, you can likely do more than this many in a week.
How do you determine which workouts are the foundational ones? They are the ones which meet the goals of your training focus for the time frame. In the general preparation phase, you're likely focusing on a weakness or general aerobic fitness, while in the later specific phase, it becomes the workouts which best represent the demands of the race.
If you're using TSS, and let's say you want to have a TSS for the bike of 190 in your upcoming 70.3, then in the general preparation phase, you are likely completing rides that are 190 TSS, but at a different intensity than what you would do in the race. As the race day approaches, you will ride more at the specific intensity for the race, at that 190 TSS. In both cases, (general and specific phases), these are foundational sessions, because it is the TSS of 190 you're building performance around.
Many athletes struggle to keep the balance of life and triathlon, and recognition of the foundational workouts from others can help athletes maintain the balance without stressing themselves out about some missed sessions here and there.
Think in terms of foundational workouts, and you will continue to build the quality sessions which are most important to your success, both now and in the weeks/months/seasons ahead.