monero-site/knowledge-base/moneropedia/paymentid.md

layout	entry	terms	summary
moneropedia	Payment ID	payment-ID payment-IDs	an optional flag that is added to identify transactions to merchants, consisting of 64 hexadecimal characters

The Basics

Payment ID is an arbitrary and optional transaction attachment that consists of 32 bytes (64 hexadecimal characters).

It is usually used to identify transactions to merchants and exchanges: Given the intrinsic privacy features built into Monero, where a single public address is usually used for incoming transactions, the Payment ID is especially useful to tie incoming payments with user accounts.

Creating a Payment ID

One can create a Payment ID quickly from the command line using OpenSSL:

# openssl rand 32 -hex