What Is The Difference Between The P2220 And P2221?

The only difference between the P2220 and the P2221 is the compensation range.

P2220: 15 to 25 pF
P2221: 10 to 25 pF

The P2221 will always work as a replacement for the P2220 but the P2220 will not always work as a replacement for the P2221. For example, the MSO2012 (input capacitance of 11.5 pF) should not use the P2220 because 11.5 pF is not within it's range of compensation. The TDS2024B (input capacitance of 20 pF) can use both the P2220 and P2221 because both probes can compensate for 20 pF.