Why types of growlers/squealers can we fill?

Growlers and squealers poured in our store must meet the following specifications:

1. Not be more than 2L in volume.

2. Have an opening that matches the size of the bungs on our filling tubes.

3. Have markings and information with regards to volume i.e. 1.89 litres or 900ml. This is a legal requirement.

We can fill other breweries/independant retailers growlers/squealers but they must meet the above criteria.