Optional or mandatory descriptors for an action?

[michalporeba]

Is it possible within current specification to distinguish descriptors which must be provided for an action or those that may or should?

I would like to use ALPS to automatically make requests to available actions using information available in the context of the client. I would like to allow some parameters to be optional, so the client can provide them if they are available, but still be able to make the call if they cannot be provided.