This is a set of options available for a given parameter.

For example, the shape parameter of the ELEMENT_SET command accepts the following values: four_node_tet, five_node_pyramid, six_node_wedge, and eight_node_brick. Hence, you can write:
ELEMENT_SET( "channel" ) {
 shape = eight_node_brick

Many enumerated values have abbreviations. For example, eight_node_brick may be abbreviated as hex8.