A ‘per diem’ is a daily payment. In this context it’s a payment offered to songwriters and composers invited to attend songwriting sessions organised by record companies. These payments are intended to cover daily out-of-pocket expenses such as travel, meals and other basic costs, so that you are not financially disadvantaged for taking part in the session.
Songwriters have often expected to attend writing sessions without upfront payment, shouldering personal costs with no guarantee of income. This scheme helps address that.