In December 2001, the International Organization for Standardization (ISO) published the first PDF/X standard PDF/X-1a (ISO 15930-1:2001).
It is based on PDF 1.3 (no transparency) and allows CMYK and spot colors. This standard has been regularly extended and is still valid today.
Unfortunately, many users are stuck with PDF/X-1a (resp. PDF/X-3). They ignore the fact that technologies in artwork production have evolved in recent years (e.g. transparencies, layers, etc.). This is why the PDF/X-4 standard (ISO 15930-7:2008) has been in existence since 2008. This was slightly revised in 2010 (ISO 15930-7:2010) and allows native transparencies, among other things.