as in "first from the left" or "first from the right" when describing people in a photo.
Likewise, "first century AD" or "first century BC" are relative to the agreed upon start point.
If you were to number the floors of a building like so:
-2: sub basement
0: ground floor
1: floor above ground floor
2: floor above that
3: floor above #2
Which one is the first floor? Is it the ground floor, #0? According to what I said in the other post, the first floor would be -2, the sub basement. However, you could use the word "first" in a relative sense: "first floor from the ground" (#0), "first floor above the ground" (#1), "second floor up from the ground" (#1), "first floor below ground" (-1), "second floor up from #2: " (#3), "second floor above #2: " (#4), etc... But unless so qualified as being relative, I would say that "first" should default to its absolute meaning of the first in the sequence (#-2). Also, using it in a relative way would leave quite a bit of room for confusion unless it is made very clear what it is counted from.
I guess I've gotten a little off topic. This stuff might have been better in distractions, but it started as a response to the original post... :-P