Generally, callback methods require access to the properties of the resource. The RMAPI provides both shell commands and C functions that you can use in callback methods to access the system-defined and extension properties of resources. See the scha_resource_get(1HA) and scha_resource_get(3HA) man pages.
The DSDL provides a set of C functions (one function for each property) to access system-defined properties, and a function to access extension properties. See the scds_property_functions(3HA) and scds_get_ext_property(3HA) man pages.
You cannot use the property mechanism to store dynamic state information for a data service because no API functions are available for setting resource properties other than Status and Status_msg. Rather, you should store dynamic state information in global files.