Credit invoice layout and numbering at Tools → Customization → Credit Template.
Tools → Customization → Credit Template (/tools/customization/credit) sets how credit invoices look and how invoice numbers are built.
Who can open it: users granted Credit Template.
Templates can differ per billing identity.
Edit visible fields, print options, and invoice id parts. Draft preview shows the result.
New credit bills use this template. Existing invoices keep what they stored unless you regenerate.