In GeoEvent Server, I have the option to register Portal. What is the difference if I federate GeoEvent Server to Portal and register Portal to GeoEvent Server? Or I should do both?
I am using ArcGIS Enterprise 10.5.
You appear to be mixing terms. GeoEvent, ArcGIS Server and Portal are different products. Only ArcGIS Server gets federated with Portal. The federation process simply hands over the security of ArcGIS Server to Portal.
GeoEvent requires ArcGIS for Server in order to function. So, lets assume you have ArcGIS Server and GeoEvent installed on the same machine. GeoEvent uses ArcGIS Server's security store to login. You have the option then, to federate ArcGIS Server with Portal for ArcGIS.
This means, Portal will manage the logins for ArcGIS Server and GeoEvent. In order to log into either ArcGIS Server or GeoEvent, you will have to provide credentials from the Portal.
In GeoEvent Manager, under Site/DataStores, you can register Data Stores. These
Data stores provide GeoEvent Server with a list of locations where source data for inputs, outputs, and GeoEvent Services originates. Folders and ArcGIS Server connections are examples of data stores that can be registered with GeoEvent Server.
You can also register Portal for ArcGIS in the event ArcGIS Server isn't federated with Portal. I believe once you federate, Portal will show up as a datastore automatically in GeoEvent. ArcGIS Online can also be added as a DataStore.
I hope this helps.
Geoevent as the extension of the ArcGIS for Server, by default, it uses the ArcGIS for Server credential. If federate the Geoevent server to portal. Then it uses Portal Credential not ArcGIS for server anymore.
If register portal to GeoEvent Server, then Geoevent Server could access the services or items from the portal directly.