Here are some of the terms used in this topic.
hot replica: A tablet replica that is continuously receiving writes. For example, in a time series use case, tablet replicas for the most recent range partition on a time column would be continuously receiving the latest data, and would be hot replicas.
cold replica: A tablet replica that is not hot, i.e. a replica that is not frequently receiving writes, for example, once every few minutes. A cold replica may be read from. For example, in a time series use case, tablet replicas for previous range partitions on a time column would not receive writes at all, or only occasionally receive late updates or additions, but may be constantly read.
data on disk: The total amount of data stored on a tablet server across all disks, post-replication, post-compression, and post-encoding.