It is the limit given on number of items that you list.
They match your feedback+DSRs and number of items that you list and (occasionaly) implement selling limits on your activity. Their algorithm is very complicated. No one can easily make out on how they calculate these things. Needless to say, eBay is the second best known for its strong-algorithm next to Google.
Your limits may be lifted if some of your items gets closed/ended or if you get some number of positive feedback in a short while.
Indraneel, correct me if I'm wrong!
Thank U!
Faheem