I’d say level=X;Y rather than level=X-Y, as stairs typically connect exactly two levels, rather than an entire range. That especially becomes relevant when the connected levels are not consecutive numbers (somewhat rare, but does happen).
I was thinking about terraces, also on roofs, or courtyards. indoor means enclosed space but building related can be outside as well.
The level tag of a highway=steps way does not tell you which levels the steps connect, just which levels the steps are on. So if there are steps to connect levels 3 and 5, which do not connect to anything on level 4 and therefore cannot be used to access level 4, but of course still physically pass through level 4, they should be tagged level=3-5 (or level=3;4;5).
That makes a range the most natural choice for their level values, even though you can equivalently use a semicolon-separated list.
The same is true for rooms. For example, a room may be two levels high and therefore be tagged as level=3-4 or level=3;4 even though it is only accessible from level 3.