This blue plastic pipe is sometimes used for the main water service line and was commonly installed during the 70’s and 80’s.
Unfortunately, this material turned out to be sensitive to the chlorine used in municipal water. Because your inspector cannot see pipe that is buried, only the visible part can be determined as functional.
Due to the expense of replacing the buried pipe, it is still in service in many homes. The pipe typically falls under an engineering standard ATSM D2666-964.
If you have this pipe in your home, you should watch for: unexplained high water bills, puddling in the yard and evidence of leaks. It would be wise to budget for complete replacement of pipe as it has been implicated in several class action lawsuits.