"Tank" is an interchangeable term with "Application Mix" throughout Field Track.
Each application record is given two reference numbers:
Batch
The batch number links all the single application across all fields. For example an application of "Roundup" batched to more than one field will have the same batch number.
Tank ("Application Mix")
This number links all application applied in the same "Application Mix" across all fields. For example an application of "Roundup" and an application of "Spray Operation" can have the same "Application Mix" number across all fields. The two examples here would have two different "Batch" numbers.
You can alter both numbers to give them the same number as another application, or create a new number to take them out of the batch.
It can sometimes be an advantage to give an application a different batch number, for example, if the rate, or price/unit have altered compared to the rest of the records in the batch, removing it from the batch will stop you inadvertently resetting the figures to another application's figures at a later stage.
Edit the number to match an existing application record, or give your application a new number respectively.