
Suppose it is confusing to those unused to dog speak, think the Import Register is for dogs, not necessary rare or scarce in their county of origin but thin on the ground in the UK, so thin that an established UK standard has not yet been written by those in the fancy (Fancy - a group of people interested in a particular breed)
As Brainless says rare just means that there have never been enough of them in the UK to have been allocated CC status, some breeds move from the Imp. Reg. or the Rare onto the full register very quickly - it does seem though that often having been very keen and numbers increased rapidly once they have full status they seem to dip before becoming more stable and established.
Then there are breeds like ours that are on the full register and have been in the country for years & years but are perhaps more of an enthusiasts breed and never become over popular, and with good reason our breeders are careful not to over breed because they are not a breed for everyone and require a degree of give and take something some dog owners would not be prepared to tolerate. So most of our breeders would not dream of having a litter unless they have a good number of prospective owners who have already been vetted.