So someone has a text entry box. They fill in the data. They then have a button that says “save” . That button that ahs a custom action that says “write to” This points to the entry field and updates a row with whatever data is in the field.
A single relation to the row you want to update would allow you to use a set column action to pass the test entry value to that column through the relation.
If you are already viewing the row you want to update, then you wouldn’t need the single relation. The text entry component could point to a user specific column, and a set column action could move the user specific value to the column you want to update.