Following are the multiple ways to invoke a flow service in webMethods.

1.using invoke step within webMethods IDE
2.using Transformers in Map Step
3.using remoteInvoke builtin service can invoke flow service from other webMethods Environment
4.using service.doInvoke() in Java services
5.using webServices,consuming webService provider operations.
6.using webmethods supported protocols like http,https.
Ex: http://hostname:portnumber/Invoke/namespace
7.using Publish subscribe mechanism in webmethods
8.In BPM using CAF application,invoking the flow service through a webservice call
9.using Scheduler in webMethods
10.using Filepolling in webMethods
11.using Startup & Shutdown services