This has been covered in great detail in a thread about a year ago. I'd have a look for it, but I'm about to hit the sack.
Suffice to say that in TERPS, runway heading is just that - with no drift correction. In PANSOPS 'straight ahead' means you maintain runway track and correct for drift. The Americas, Japan, Taiwan and Korea use TERPS, the rest of the world uses PANSOPS.