In an anonymous platform context... boinc 7.6.33, linux host... seti@home project...

What is the <plan_class> tag in the app_info.xml file? Looking in the boinc wiki it says that in the plan_class tag one should fill in the "plan_class" of the application. Well, that's not very helpful. Is the plan_class built-in to the application? If so, how does one know what it is? Or, is the plan_class somehow implied in the name of the executable file?
The wiki also indicates that the plan_class tag is OPTIONAL. So, under what conditions is it required and what are the consequences of leaving it out if it is not required?
I'm really only aiming at having one cpu app and one gpu app on a given named application.

Thanks for any light you can shed on this.

I think plan_class might come handy in a situation when your host has both AMD and Nvidia gpu's. Then plan_class could be used to control them separately. I don't know about Seti apps, but sometimes a project might have an app that has multiple sub-versions for different GPUs and platforms.

Maybe the few posts here will give something useful:
https://www.reddit.com/r/BOINC/comments/4isriv/setihome_app_configxml_to_run_2_tasks_on_nvidia/

Not about Seti apps... but in Einstein@Home... here is their file that might give an example of some applications having multiple plan_class for different architectures:
https://einsteinathome.org/apps.php?xml=1

As per https://boinc.berkeley.edu/trac/wiki/AppPlan

Plan classes are policies that allow the BOINC scheduler to decide:

    whether an application should run on a particular host;
    what resources it will use (# of CPUs and GPUs, optionally memory usage)
    how fast it is expected to run. 

Each app version has an associated plan class; multiple app versions may have the same plan class. See how to specify an app version's plan class. Each plan class has a textual name. The default plan class (whose name is empty) is for single-threaded CPU applications.

You can check the application's plan class at the project's applications page, or perhaps it's shown in the name and therefore shows in the properties of the tasks. Like Seti@Home has the application name setiathome_v8 8.20 with between parenthesis the plan class, for example (opencl_ati5_sah). You can check that against their applications page.

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Thanks for the replies. The examples and other references were enough to get me on the right track. All is well now. Up and running with the cpu and gpu apps that I wanted to use. I might note (for the benefit of others who might find this thread) that the boinc manager action "read config files" is NOT SUFFICIENT to update the boinc manager process state, especially in an anonymous platform context - a boinc RESTART is required. My best analysis is that the app_info.xml file is NOT re-read and when the app_config.xml file IS re-read there can be many mismatches. That fact is probably spelled out somewhere in the boinc documentation and user guides. But easy to overlook and/or forget. :(

Correct, app_info is read at BOINC startup. app_config edits can be reread.