Explanation

In a dial-in mode, the destination initiates a session to the router and subscribes to data to be streamed. Dialin mode is supported over gRPC in only 64-bit platforms In a dial-out mode, the router initiates a session to the destinations based on the subscription. All 64-bit IOS XR platforms (except for NCS 6000 series routers) support gRPC and TCP protocols. All 32-bit IOS XR platforms support only TCP.

Explanation:
In a dial-in mode, the destination initiates a session to the router and subscribes to data to be streamed. Dialin mode is supported over gRPC in only 64-bit platforms In a dial-out mode, the router initiates a session to the destinations based on the subscription. All 64-bit IOS XR platforms (except for NCS 6000 series routers) support gRPC and TCP protocols. All 32-bit IOS XR platforms support only TCP.
Reference:
https://www.cisco.com/c/en/us/td/docs/iosxr/asr9000/telemetry/b-telemetry-cg-asr9000-61x/b-telemetry-cgasr9000-
61x_chapter_010.html#id_36445

Section: Advanced Enterprise Campus Networks

Explanation
There are two types of subscriptions: periodic and on-change. With periodic subscription, data is streamed out to the destination at the configured interval. It continuously sends data for the lifetime of that subscription.
With on-change, data is published only when a change in the data occurs such as when an interface or OSPF neighbor goes down.
https://developer.cisco.com/docs/ios-xe/#!streaming-telemetry-quick-start-guide/streaming-telemetry