Processes having a Process Start Option set to 'On-demand' are not listed on the Processes tab of that object instance so they cannot be initiated.
Release : All PPM Versions
Component : CA PPM BUSINESS PROCESS MANAGEMENT
The 'Available for On-demand Start' setting found under the Objects tab on the process properties needs to be set to Yes in order for the process to be available under the Processes tab of an object instance. If this option is set to No, the process will not be listed regardless of what Start Option it's set to.