Well, I they are the zones used by USA Cycling, and probably Friedel. Its been a while since I've pick that book up. In essence, this is the scale:
1 - Barely Trying - 60-65% MHR
2 - Moderate - 65-70% MHR
3 - Hard - 70-80% MHR
4 - Very Hard - 80-85% MHR
5 - Suffering - 85-100% MHR
You tell me if thats what Friedel uses... I generally use LT as a measure for zones 4 and 5. Zone 4 is JUST BELOW Lactic Threshold (Like what you'd want for a TT of any length), Zone 5 is in LT - can't stay there for too long (sprints, moments of very steep climbs, parts of TTs)