All mesh entities have an ID associated with them which is unique within the corresponding mesh entity ID space. For example, a hex will have an id which can be used in commands such as "list hex 17". However, this ID is only unique amoungst the hexahedra in the Cubit session. There could also be a tet, quad, tri, edge, node, etc. with ID 17.
Whenever a hex, tet, quad, tri, etc. gets put into an element block, it is assigned another ID which is called the Global Element ID. The Global Element ID is unique among all element which have been put into any block. It is exported to the Exodus file format so that downstream analysis applications can map elements back to the corresponding Cubit hex, quad, etc.
bri x 10
mesh vol all
block 1 hex all
list hex 1
Cubit> list hex 1
List element <id_range>
Users can control the assigned Global Element ID with the renumber command.