Flow and global from function not in default incited into permanent file

sorry to ask, but I may not have entered the right words in the search box when searching the forum.
I have set the variables so that I have default in memory and can also write to a file.
In change and other nodes I can decide this via a pull down where I would like to have this stored.
But I have not found a way how to do this out of a function. Is there a possibility of influencing this or does the variable always go out and in into default?

I have a temperature in the default (memory) at one point and in the file at another. These are different :frowning: because one is written to the file via change wehr I chose file but the function writes to default and reads from default memory. This confuses my data somewhat

It is describe here in docs.