Hello!
This is a spinner wheel. This spinner wheel has three naming systems for pitches and pitch classes; letter, solfeggio and number. They can all rotate freely of each other.
Maybe you're asking, "Why three naming systems for pitches and pitch classes?" I'll tell you! It's because having distinct naming systems can help with practically understanding secondary harmonic function by assigning different naming systems to different levels of harmonic function.
Watch how these examples illustrate my point:
• Re is the 2 of Do.
• Re is the 3 of Te (rhymes with May).
• Re is the b3 of Ti.
• Re is the b7 of Mi.
Watch how these examples do the same:
• D is the Re of C.
• D is the Mi of Bb.
• D is the Me (pronounced May) of B.
• D is the Te of E.
These prior examples facilitate this type of thinking and understanding more easily than these following examples.
• Re is the Mi of Te.
• Re is the Te of Mi.
• 2 is the 3 of b7.
• 2 is the b7 of 3.
Sure, all those examples are correct and essentially performing the same function as the ones listed before, but rather than fostering understanding with two respective naming sets, it risks confusion by using the same naming system for two discrete levels of functionality.
Anyway, hope that helps! Happy Music-ing!