Enabled only if allowing overrides to receivable type (option in AR Company Parameters).
Initially defaults the receivable type specified for the customer in AR Customers. If no receivable type was specified for the customer, then the receivable type specified in AR Company Parameters is used.
If a contract is entered, default will be receivable type specified for the contract (in JB Contract Info). If no receivable type is specified for the contract, default will be the receivable type for the customer or company (if none exists for the customer).
You cannot change the receivable type for Adjustment, Credit Memo, or Write-Off transactions. Field will always be disabled, regardless of how you set the 'Allow Override of Receivable Type' flag.