The "Tidy Instances of Procedures" section of the TIBCO iProcess Engine Administrator’s Guide document states 0 (zero) can used as the procedure identifier:

swadm tidy_instances machine_ID proc_ID
where:
• machine_ID is the unique identifier for the server. If you specify a value of 0, the command will apply to all servers in iProcess Engine.
• proc_ID is the unique identifier for the procedure. If you specify a value of 0, the command will apply to all procedures in iProcess Engine.

Using zero as the procedure identifier results in the command failing:

swadm tidy_instances 1 0
Usage: TIDY_INSTANCES { <node_id>|? }  { <proc_id>|? }
Description:
        Deletes old instances of a proc according to value
        of attribute PROC_VER_NUM_INSTANCES.
        node_id is typically value 1.
        proc_id is a numeric value >0.