PDF/X is a subset of the PDF ISO standard. The purpose of PDF/X is to facilitate graphics exchange, and it therefore has a series of printing related requirements which do not apply to standard PDF files. For example, in PDF/X-1a all fonts need to be embedded and all images need to be CMYK or spot colors. PDF/X-3 accepts calibrated RGB and CIELAB colors, while retaining most of the other restrictions of PDF/X-1a.
