Help with adding transaction tracking data

The php video web script that I’m trying to modify has a process where Users generate credit units from viewed videos.
The total ‘balance’ of credits is stored in mysql db table named User, in a column titled “balance”, with each total reflected in a users’ row, and the balance total gets update (goes up or down), depending on the activity of the credits.

However, if 2 credits get added to the balance, for example, there is no tracking of what type of activity caused the balance total change.

What should be modified?
Should the modification be another row in the User table?
Or should a separate table that gets a new row inserted for each transaction that affects the amount be created?
What else might this need?
I look forward to all comments/suggestions.

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Thanks for your reply.
Can you give me an idea of how that’s accomplished?