In position history can you view positions in a particular asset, this is used when you need to find which day a difference has occurred. The internal accounts (Internal Balance) are compared to internal custody accounts.
The default period is one month back in time, but this can always be changed, simply use the date picker or the arrows. Change the date period with the arrows above the date filter to find the day when the diff occurred.
If you want to investigate which transactions were made on a particular day to find the difference, click on the house icon to investigate internal transactions.