POSIX is mandated by government contracts - and so are many other things. If you’re selling into governments you need to be able to sign off that you comply with the appropriate standards (whatever the government wants - usually a standard written by the vendor they want to choose)..
One of the things with government regs that I've learnt working in biotech is that access to the document(s) isn't enough. There are various nuances with the interpretation, and only someone well versed and experienced in them can prepare you for an audit. Our company pays top-dollar for these consultants.
