The user command tables and the site command tables are optional to use. They must be defined for your installation and present when ISPF is initialized. To make use of them, you must update the ISPF configuration table to include their application identification.
ISPF uses a specific order when searching the tables for commands you enter. However, you do have some control over the search order when using the optional site command tables. The site command tables can be searched either before or after the system command table. To define the search order relative to the site and system command tables, update the ISPF configuration table.
When you enter a command, the application command table is searched first. If the command is found, no further searching is necessary. If the command is not found in the application command table, up to 3 user command tables are searched. If the command is not found in the user command tables, up to 3 site command tables are searched or the system command table is searched (depending on the search order defined in the ISPF configuration table). Finally, if the command is still not found, the remaining command table(s), site or system, are searched. User command tables and site command tables are only searched if they have been defined in the ISPF configuration table and are present at ISPF initialization.
See The ISPF Configuration Table for more information about modifying the ISPF configuration table.