Hello
It is related to the timestamps of the events. In a normal situation where all the events are beign received one after another in the timeline, you would have only one hot bucket, and it will be filled until it reaches the maxDataSize limit.
But, you could get events from the past that won't "fit" in the time frame of the existing hot bucket, and in that situation Splunk will create a new hot bucket for the events coming from that "past time frame", in this case you will have 2 hot buckets.
So, hot buckets groups the events by time.
Regards
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