The Modification log button is available if you have configured that modifications shall be logged, in the Log Selection procedure under the Printers / Users in the Global Settings module. You can then access a window via this button where you can see what has been modified for the purchase order in question. If a record has been deleted you will not be able to view information about this here. In this case you should instead use the List Deleted Data procedure in the Global Settings module.
In the window you can view who made a modification and when, and also what has been changed. You can see which field values that have been changed, with the value before and after the modification. An identifier is displayed when a field value can be changed from several different places. If such a field value has been modified, the identifier shows from where the change has been made.
It is possible to filter in order to only display the modification log for a certain field. This is useful if there have been many modifications in different fields, you can then choose to only see the changes in the selected field.