You might simply say that "low HD monsters are on upper levels of a dungeon, high HD monsters are on lower levels."
However, OD&D (1974) had a chart where they divided monsters into six categories ranging from easy to hard. An easy (type 1) monster might be something like kobolds or orcs, whereas a hard (type 6) monster might be something like a dragon or spectre. On level 1 of a dungeon, a random monster would be a type 1 on a roll of 1-2 (on a d6), type 2 on a roll of 3-4, type 3 on a roll of 5, or type 4 on a roll of 6. So, a starting party might encounter a type 4 monster (ogre, wraith, gargoyle) on the first level of a dungeon.
DCC's intent is to use modern rules to simulate much of the experience of the good old days. The secret is that in the good old days, you might find something nasty close to the surface and had to learn to judge when to fight it or when to run away from it. Modern RPGs tend to have this "if it exists, I must be able to kill it" because of a metagaming game-balance assumption.
DCC makes no such assumption. The DM can place anything anywhere.