A CDR is a conditional route. This is an airway or an Upper Air Route which is only available at certain times of the day and when certain military areas are not active.
For example, there are several conditional routes which transit the North Sea which are only available when the fast pointy military jets are not playing in their military airspace. As I recall from many years ago, D323 was said danger area and when this was active several of the conditional routes were unavailable.
This is an extract from the UK AIR ENR section which details a little more about the CDRs.
Routes are used as described below.
(a) Category One — A route which is permanently plannable during the times published in ENR 3.
(b) Category Two — A route which is only plannable in accordance with the conditions stated in the daily Conditional Route
Availability Message (CRAM) issued by the Central Flow Management Unit (CFMU).
(c) Category Three — A route which is not plannable per se but may be used tactically at the discretion of ATC.
A CDR may have more than one Category.
In principle, the military have to plan their activity at least 24 hours prior to the activation of the danger area. Conditional routes are closed as per the booking and, as described above, the CFMU publish to operators which CDRs are closed and which CDRs are available.
Hope this helps a little bit.
Kind Regards
Doody
