When a Custom Dimension is active, which data will it affect?

Custom Dimensions are applied at the hit level. Once a Custom Dimension is activated and configured, it will only collect and associate data with hits that occur after its implementation. This is because Google Analytics processes data in real-time or near real-time as hits are sent. It cannot retroactively apply a new dimension to historical data that has already been processed and stored without that dimension. Therefore, any data collected before the Custom Dimension was applied will not have that dimension associated with it. It will not affect all data in the property, as historical data remains unchanged, nor will it affect none of the data, as future data will include it.