The syssegments table, one in each database, lists the segments in a database.
A segment is a collection of the database devices and fragments available to a particular database. Tables and indexes can be assigned to a particular segment—and therefore to a particular physical device—or can span a set of physical devices.
create database makes default entries in syssegments. sp_addsegment and sp_dropsegment to add and remove entries from syssegments.