What's the difference between

PMI is a SDO (standards developing organization) for US ANSI (US national standardization body). As such PMI standards can become ANSI standards. Part 2 of the PMBoK Guide ed 6 is such a standard.

The PMBoK Guide, The Standard for Program Management, The Standard for Portfolio Management, and Organizational Project Management Maturity Model (OPM3) have all been approved by ANSI as American National Standards.

PMI was also involved in the development of ISO 21500, on behalf of ANSI (only national standardization bodies can work on ISO standards), and contributed its standard (PMBoK ed 4) as input to the ISO current standard.

Page 40

1.1.1 The Standard For Project Management
The PMBOK Guide is based on The Standard For Project Management