What I am trying to achieve is to avoid mixing items with different expiry dates in a location.

Same item, different batch, same expiry date OK
Same item, different batch, different expiry date NOK.

The Parameter "Number of days to mix batches" accepts any numeric value, including zero.

When a value greater than zero is entered the system will ensure that the difference between the expiry dates for all batches on the location are not more than the number indicated in the "Number of days to mix batches" field.

When zero is entered, the system ignores this control and allows any batches to be stored at the location, regardless of the expiry dates.

I suggest that the value "0" (zero) means that only batches with the same expiry date can be put at this location.

The "Override rules for batch days" on location profiles allows this to be overridden if required.