If you are using a measure as filter in the business transaction then you will have to define a threshold which is minimum value is 1.
Since filters are based on measures, you need to configure thresholds in the measures you are assigning for filtration.
The value 1 threshold is all about the occurrence which is use in the BT filtration.
Please keep in mind that the threshold settings of measurements are used both by the Business Transactions filter and by incidents. It is recommended that you create separate instances of measures to use in Business Transactions.