Hard/cold tyre. The SC0 is the soft, hot weather tyre, SC1 is a medium, and the SC2 is harder, for cold weather and/or abrasive surfaces. The X is beyond the 2, but I haven't seen a Pirelli compound guide that includes the X. Previously there was only a PI compound, but according to the homologation list, there's a different X compound for other tracks.
For the rears, Pirelli do their compounds like Dunlop. Soft is for hot weather, hard is for cold.
The reverse for the front:
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