Beam model contract for finalization of CheckpointMark's

Question: What is the contract in Beam model for when checkpoint marks shall be finalized, is there any ? 

I'm working on pipeline that reads messages from Kafka using KafkaIO, and I'm looking at commitOffsetsInFinalize() option, and KafkaCheckpointMark class.

I want to achieve at-least-once message delivery semantics and want to be sure that offsets committed to Kafka after they are written to some sink.

Looking at interface of CheckpointMark  it's not clear when finalization shall be expected to happen.

Is it runner dependent, what to expect when executing on DataflowRunner ?

And reading KafkaIO.Read javadoc on commitOffsetsInFinalize 

https://beam.apache.org/releases/javadoc/2.9.0/org/apache/beam/sdk/io/kafka/KafkaIO.Read.html#commitOffsetsInFinalize-- 

also doesn't bring clarity to my understanding, particularly the phrase 

But it does not provide hard processing guarantees

What exactly are hard processing guarantees ?

Can I ask, please for documentation improvement in respect of CheckpointMark and commitOffsetsInFinalize.