In reply to TobyA:
From Strava site:
"To decide the category of a climb we multiply the length of the climb (in meters) with the grade of the climb in percent. If that number is greater than 8000 then it is a categorized climb. The minimum percent grade must be 3% or higher. So, for a 4% avg grade climb, it has to be 2km at least to be categorized. Cat 3 = 16000, 2 = 32000, 1 = 64000, HC > 80000. "
"Strava segments are automatically categorized as a climb if the data fits our criteria. The categories are 1-4, with HC being above a category 1 climb, and HC being the hardest category of all. Strava segments can be generated two ways - as a user, you can create a segment from any data on the site. If the the data you choose for a segment fits into a categorized climb, you'll see a red icon on the finished segment with either the number 1-4, or HC. If you are the riding new territory where no Strava data has previously been recorded, Strava will auto-detect and auto-create categorized climbing segments for you. "