Technical Terms

AS-Set:

An AS-Set in the IRR (eg. RIPE DB) contains a list of members: AS-Sets are used to build filters for upstreams and IXPs.
The AS-Set contains your Downstream ASNs and your own ASNs.

Prefix Count or Prefix Limit:

A Prefix Limit is usually found on IXPs and Transit signup forms. It indicates the number of prefixes you plan to announce + some margin.
For example if you have a /44 and announce 4 prefixes it would be recomended to set the Prefix limit to 10 or 20. Most IXPs will reject a too high Prefix Limit.