Domains are tuples, the structure is (field, operator, value).
Field is the data in the database,
Operator is the comparison, = equal, != not equal, > greater than, < less than, etc.
Value is the data you want to compare with the field, it can be another database field, a constant or a calculated value.
Typically when you send a domain into a function it is sent as an array so it must be contained with in brackets [ ] whether you have 1 or more tuples.
The default operator between tuples in the array is & (AND), to modify that you can use | to indicate OR
So in your example the domain is
translated in to human:
orders with the same user id OR a blank user id will be part of the domain.
Without seeing the rest of the code a better answer cannot be given on what exactly the domain will be because user.id is not known from that code alone.