How does the checkpoint get upgraded in Re:Zero? Spoiler Alert

As we have seen in Re:zero, Subaru wakes up in a certain palace after he dies, which is a checkpoint. As the story progresses, we see the upgrade in checkpoint as in normal games.

In most of the games, checkpoint is upgraded (in case of Auto) when you reach certain places or after you've completed certain tasks. The gamer is notified or knows when checkpoint is upgraded; it was not in Subaru's case.

The checkpoints are:


In front of shopkeeper
Bed, in the palace where Emilia lives
The big tree, where they killed the white whale


The checkpoint is upgraded after the task has been completed. But that was not the case, when Rem was saved from the dogs from the forest, it was a task that has been completed but when Subaru returned from death, in the witch cult arc the save point was in front of shopkeeper but not the palace's bed.

From this, a task is not the only thing that triggers the upgrade, but place also plays a role. If that is the reason then the place where big tree is around should have been the save place because Subaru passed that tree but did not stop there.

From all the conditions, checkpoint upgrade not only depends on place and task but other things also. These conditions are just the guess or conclusion derived from above cases. And Subaru also doesn't know how upgrade works and upgrade checkpoint manually, like going some place and doing some thing, e.g talking with the shopkeeper.

How does the checkpoint get upgraded, and what are the conditions required for upgrade?
It seems that in the first safe area after all dangerous events have passed and subaru has no chance to be harmed it creates a save point, however it doesn't update at every moment he is not in imminent danger.