I remembered reading this on the RPA website when I was applying for my CPH number, it seems that it is all down to distance the land is from your registered holding address
A5. What is the 5/10 mile rule for CPH allocations?
The five mile rule applies to all sheep and goat keepers and simply means all land within a five mile radius of the main Place of Business is covered by one holding number, it doesn’t need a separate CPH number. The five mile distance is measured as a straight line from the main site and any outlying land that is within five miles of the main site (either fully or partially) is all considered to be the main holding. Only land outside this boundary needs a separate CPH number.
The ten mile rule applies to all livestock keepers, excluding sheep and goat keepers. The ten mile rule works the same way as the five mile rule.