5.3.4.3 Receiving to a subaddress
Address:
> If the equality X==Hs(pV0* R)G+PSi is true

Subaddress
> if the equality PSi == X - Hs(pV0 * R)G is true

https://github.com/monerobook/monerobook/blob/master/chapters/5.md#5343-receiving-to-a-subaddress

What is the sense of different versions of the same equality?