What do "level" and "difficulty" refer to?

I've been playing Warframe for a bit now and I still can't figure out what I should be expecting with regards to mission levels and difficulties.

I'm talking about the level that you see on the planet map for mission selection, like this:

And I'm talking about the difficulty you see on the mission loading screen, like this:

I've been thinking perhaps the level is how hard enemies are and the difficulty how many there are, but I haven't really managed to figure out a pattern. What do they mean?

Solution 1:

The level of the mission dictates the levels of enemies (or the levels they start at for endless defence). Bosses and nightmare mode's enemies are higher level than the mission level, but they also depend on it. Bosses in lower level missions will be much lower level than those in Pluto/Europa.

Difficulty seems to refer to the amount of enemies.

Solution 2:

Level refers to the level range of spawned enemies (not my idea, it's 3ventic's, but I'll back him up), difficulty refers I think just to how purely difficult the mission is, usually certain gamemodes are harder than others, but there is quite a bit of variation. Also, I use the level to predict what rate of gear I need. It's kind of subjective, but for every 10 levels on the mission, I would use gear with 3-5 levels more if I have 2-3 other players with me.