Procedure for Deriving Payment Addresses
The method for deriving child keys from extended keys is specified in BIP 32.

Note that the specific derivation paths for xpubs, ypubs, and zpubs are not defined by BIP 32 but rather are constructions created by Electrum and SatoshiLabs. These have a specific meaning described in SLIP 132. Their encoding is the same as described for xpubs in BIP 32 but with a different prefix byte.