Can't monitor original estimate in Jira Automation rule

[Wim Matthijs]

I want to create a rule if somebody would change the original estimate of a story, that the total estimate of the parent epic gets updated as well.

However when i create a trigger that checks on updates  of issue fields i can not select the original estimate field, i can only select the initial estimate field.

i tried googling for the difference between these two fields but it came up dry.

Am I missing something here?
Is there some information available about the difference of these values?