PDF/X-1a:2001 and PDF/X-3:2002 are designed to provide a reliable and stable PDF for printing purposes: requires all fonts to be embedded, flattens transparency (this is important for effects such as drop shadows to render correctly).
PDF/X-1a:2001: allows CMYK and/or spot colors*
PDF/X-3:2002: allows CMYK, RGB*, or spot colors*
We recommend using PDF/X-1a:2001: or PDF/X-3:2002: for all files submitted.
*Spot colors, RGB, and ICC profiles can produce unexpected color results when printing. Any dissatisfaction with color shift will be the publisher's responsibility to correct.