I'm going through specification of Specialized Epic Comp bicycle (link) and there is in column FRONT DERAILLEUR: "Custom SRAM, 2X10, high direct mount". What does it mean? Is it some kind of specific derailleur for this bike? Does it mean it is not possible to use ordinary SRAM x7, x9, x0 series? Thanks in advance
Your bike uses a "high direct mount" front derailleur - so yes, you can upgrade to an X0, as long as it's a high direct mount model.
SRAM and Shimano both sell "non-group" transmission and braking components, usually only to OEM customers (i.e. the bike manufacturers). This means they aren't in a "group" (e.g. X7, X9, XX1, Deore, SLX, XT, etc) but allow bike manufacturers to hit a certain price point. For instance I've got some brakes and hubs that're roughly equivalent to Shimano Deore components, but they're fairly plain looking, with only a Shimano logo on them. They're perfectly good parts - just not quite as high end / refined as as the ones that get the nice branding.
If you look closely at the zoomed photo on Specialized's site, you can see that the label on the "custom" front derailleur says only "SRAM" and "2x10". The regular ones made by SRAM say "X0" or other model names on them, e.g. here: http://www.competitivecyclist.com/sram-x0-2x10-high-direct-mount-front-derailleur-srm0437
It appears that the style of mount is readily available, but perhaps not the precise model with no labeling that Specialized is using. Maybe that means they're using something "between" the regular SRAM grades, or maybe a different surface finish, or.... But this shouldn't matter if you simply need to buy a replacement.