Customs procedure codes denote which procedure the goods are moving into or out of. For CDS, the codes are split into two parts. There is a single four-digit procedure code which is then combined with one or more three-digit additional procedure codes (APC).
The required links for procedure codes and additional procedure codes are accessed through the tariff: https://www.gov.uk/government/collections/uk-trade-tariff-volume-3-for-cds--2
Exports
The list of four-digit procedure codes can be found here: https://www.gov.uk/government/publications/appendix-1-de-110-requested-and-previous-procedure-codes
Select the first 2 digits, which denote the intended use, then select the procedure required. Each four-digit code has its own completion rules, stating the conditions for using that code, as well as the requirements for using that code on a customs declaration.
Once the first part of the procedure code is confirmed, then the additional procedure codes can be chosen.
https://www.gov.uk/government/publications/appendix-2-de-111-additional-procedure-codes
Note that not all procedure codes and additional procedure codes can be used together, therefore the correlation matrix provided by HMRC should be referred to:
Inventory Exports
There are separate tariff sections and separate navigation guides for inventory exports.