Hello! Thank you, I was not aware of those tags. The reservoir:* tags are not leftovers from an import, I'm intentionally adding them.
These are the meanings in the datasource I'm using:
- reservoir:capacity - total capacity of the reservoir, in m^3.
- reservoir:active_capacity - useful capacity of the reservoir (excluding the dead storage volume of the reservoir), in m^3.
- reservoir:max_level: Maximum water elevation during flood periods, measured from sea level
- reservoir:full_level: Maximum water elevation that the reservoir should be kept at most times, measured from sea level. This should be some meters below  reservoir:max_level, so that the reservoir should be able to accomodate a flash flood.
- reservoir:min_level: Minimum water elevation that allows the reservoir to fulfill its purpose (be it irrigation, power, etc.), measured from sea level.

So I think ele:max and ele:min are better alternatives to reservoir:max_level and reservoir:min_level. ele:natural is not the same as reservoir:full_level, since ele:natural describes the elevation of a body of water if there was no dams/weirs, which is not the case.

So if you could suggest better names for reservoir:capacity, reservoir:active_capacity and reservoir:full_level, I'd be very thankful.