One thing to understand is that PMI is just providing a Framework, a guidance for Project Managers. Companies do not follow the process groups and knowledge area to the letter, moreover they tailor the processes to their methodologies. PMI even stress that you use whatever processes are appropriate for your project, and then tailor them to your needs.

More commonly some of the big consulting companies tend to use some version of the PMI framework, and I've also come across US government contractors that tell me the same.

There is no specification given by PMI for project management. PMI only says that you must follow ethics and professional conduct, and nothing else.

The project management guidelines what you read in PMBOK is a type of globally accepted approach. Project management is not made to win award of purity by following process. What matters more in any industry is deliverables and every organization has its own process, policy and approach.

When you see project management used in bigger companies at a tree view, you will find that the practices do follow PMI made concepts. If you have read PMBOK nicely, you must recall that before PMP chapters begins, PMI has said very clearly, the concepts mentioned here are something like best practices and not something that has to be followed.

Saket, PMP