A track's UID is its Unique IDentification number. Please note TW2 has used new UIDs due to the discrepancy between the location of start points - this meant there was no easy way to correlate the two sets of data with a high level of accuracy in an automated fashion. However you can search using either the old or new reference number.
The best way to think of UIDs is simply as a method of naming track sections. The UID provides a single, consistent way of numbering track lengths that can be calculated from the position of that track. Remember that any route, way or green lane may be made up from a number of track lengths, connecting together at junctions or other significant points.
The UID consists of two parts; a two letter and four digit geographic ID, and a two digit suffix. The geographic ID part is a simply derived system of numbering lanes, that can be calculated by TrailWise from the start point of a track. It is simply the OS reference for the 1km grid square containing the most westerly or southerly end of the track; example if a track runs east from grid ref ST 235 787 its UID would begin ST2378.
More than one track section may start within the same 1km square, so a two-digit number is appended to the geographic ID to make sure it is truly Unique, to give the full UID - example ST2378-01.
Although the UID is based upon the Ordnance Survey National Grid System, it does not fully describe the location of a route to a sufficient extent to establish where both the start and end of that route are located.