I've never heard of L1, L2, L3... convention, and I suspect it is peculiar to that company - it is the kind of thing that ought to show up in the company's Organizational Process Assets.
The closest thing that come to my mind is that in the capital investment control system (ECPIC), the US Government records three dates for every event.
- Planned - the originally planned date for the event
- Projected - the current best estimate for the event
- Actual - The actual date of the event
On the other hand, ECPIC largely ignores charters, while the system you're describing (L1, L2,...) seems oddly rooted on charter date. Given that the charter is at the open end of the cone of uncertainty, I'm not convinced of the value of tracking schedule variance at that granularity that early in the project. I'm not skeptical - I've seen some studies that assert that you have a 60% confidence in project success at roughly the charter milestone, and that might align with the system you describe. But I'm not yet convinced.