There is no universal rule to follow when coding ISO documents.
I can only tell you about my practice.
ISO 9001:2015 invites us to follow the process approach. So, I model how the quality management system works, as a set of interrelated processes, and I code each process with a number and a verb+noun. Like:
4.Develop new service
Now I use that numbering as the basis for coding the documentation. Like, for example:
PD1.0 – Process Description 1 version 0.
Any work instruction related with Process Description 1 is coded as WI.1.1; WI 1.2; …
Forms, I code them with a serial number.
The following material will provide you information about the document control:
- ISO 9001 – How to structure quality management system documentation - https://advisera.com/9001academy/knowledgebase/how-to-structure-quality-management-system-documentation/
- ISO 9001 blog – New approach to document and record control in ISO 9001:2015 - https://advisera.com/9001academy/blog/2015/06/30/new-approach-to-document-and-record-control-in-iso-90012015/
- Free webinar on demand - Free webinar – How to use a Documentation Toolkit for the implementation of ISO 9001 - https://advisera.com/9001academy/webinar/how-to-use-a-documentation-toolkit-for-the-implementation-of-iso-9001-free-webinar-on-demand/
- Free online training ISO 9001:2015 Internal Auditor Course
- Book - Managing ISO Documentation: A Plain English Guide - https://advisera.com/books/managing-iso-documentation-plain-english-guide/