SRD 5.1 will not end up under the Creative Commons in total. Only part of it will, and likely that will be very small, though we do not yet know the extent of it.
I mean, they list the pages on the document. So we kind of at least know their intent.
The core D&D mechanics, which are located at pages 56-104, 254-260, and 358-359 of this System Reference Document 5.1 (but not the examples used on those pages), are licensed to you under the Creative Commons Attribution 4.0 International (CC BY 4.0).
(from their draft for discussion document)
We skip all of the classes and races to get to page 56.
Pages 56-104 has sections that are under the headings "Beyond 1st Level" (Which beyond advancement also includes multiclassing, alignment, languages, inspiration, Backgrounds), "Equipment", "Feats", "Using Ability Scores","Time", "Movement","The Environment", "Resting", "Between Adventures", "Order of Combat","Actions in Combat","Damage and Healing","Mounted Combat", "Underwater Combat", "Spellcasting" .
Then we skip all of the spells. We also skip over the rules for Traps, Diseases, Madness, Objects, Poisons, and Magic Items.
(I have no idea if the skipping of the rules for Traps, Diseases, Madness, Objects, and Poisons is deliberate or accidental due to the quasi-random organization the 5.1 SRD has. It's clear that the spells and magic items are deliberately left out tho).
Now we're at pages 254-260 which is Monsters, and covers the general terminology of a monster stat block.
We skip starting at page 261 - which is where the monster descriptions start - to pages 358 and 359, which is the summary of Conditions.
So it's the basic core rules minus the rules for Traps, Diseases, Madness, Objects, Poison, and the general rules for how Magic Items work (which again might be an oversight rather than intentional). No classes, no races, spells, no magic items. Just some but not all of the mechanics. The six basic ability scores are listed under "Using Ability Scores", so at least we're safe from them deciding to sue someone because they used the six common D&D ability scores.
BTW - I didn't notice this yesterday but it specifically excludes "examples" from the Creative Commons release. So they're being really precious about "protecting" the one Feat that is in the SRD (Grappler) and the one Background that is in the SRD (Acolyte) from nasty people who would use their examples in violation of their morality clause.
EDIT: And in case it's not clear, the core mechanic of roll a d20, add a number, compare to a difficulty is in the "Using Ability Scores" section and also in the combat section, even though you might think the core mechanic of the game would be on page 1 of a reference document.