AT, in this case, stands for
auto-substitution. This means that, if there are source segments that contain only
placeables, such as
alphanumeric strings, their corresponding target segments will be automatically filled in with the placeables:
These can be established in the
TM (Translation Memory) settings:
If you don't want to have any placeable, you can remove all of them by clicking the one you need to remove and then selecting it from the drop-down list: