ISO 9001

Basically a description of the processes you employ. NB the shorter the better; not all processes need to be described o be compliant.

https://www.iso.org/standard/62085.html

ISO 9001 is a globally recognized standard for quality management. It helps organizations of all sizes and sectors to improve their performance, meet customer expectations and demonstrate their commitment to quality. Its requirements define how to establish, implement, maintain, and continually improve a quality management system (QMS).

Implementing ISO 9001 means your organization has put in place effective processes and trained staff to deliver flawless products or services time after time.