There are many times where you have to change an approved mapping. When you reset it, unless you have selected the option, blank it out, it keeps the values in Action, Source,Field, Target Relationship etc.
When you change the mapping, specially when is a Rule where you do not pick a table and field from the drop down, you can easily forget that you had a Target Relationship in the vertical view.
When you approve and regenerate the rule, DSP is not going to generate an error (and it shouldn't), so you are not aware of that joining.
This is specially dangerous when the inner join make that the source rule is going to update less number of entries than expected.
Let say I had a mapping that is a Rule, UPPER(UserID) and this user is coming from a different table so I created a Target Relationship. My source table have 100 entries but there are only 80 users, so only 80 rows are going to be updated.
If I change the mapping to Default Victor, where I want to make my self the default user for all entries, I might not remember that I have to remove the Target Relationship from the vertical. Then, when I create the Rule, it is going to still update only 80 rows instead of all of them.
You should realise later on when you review your file, but in cases more complex where you have a source rule and a target rule..you might not notice.
It would be useful that we can identify in the Hor view that we have selected a Target Relationship.