If it has a subcategory like 'Ford', then it would be 01 (the parent category) and then'Ford' which we will give 01.
UDM_LIMIT_CAT - defines document category limitations. Categories are similar to tag feature, but nested. So you can have one category inside another and so on. You have to use two characters for each level. Use a hex number going from 0-F or a 36 base number going from 0-Z. Therefore a top-level category like 'Auto' would be 01. If it has a subcategory like 'Ford', then it would be 01 (the parent category) and then 'Ford' which we will give 01. Put those together and you get 0101. If 'Auto' had another subcategory named 'VW', then it's id would be 01 because it belongs to the 'Ford' category and then 02 because it's the next category. So it's id would be 0102. If VW had a sub category called 'Engine' then it's id would start at 01 again and it would get the 'VW' id 02 and 'Auto' id of 01, making it 010201. If you want to search for sites under that category then you pass it cat=010201 in the url.