You may suspect that setting up a domain name in the field catalog will make some effect on the display, for example in data type, length, lowercase sign, but it seems that it has no effect in ALV grid. Maybe just for information and for Report-Report interface, where this field is use as one of the matching criteria for selection-screen parameters.
As you can see on the next image, although I gave the DOMNAME value FLAG, it didn’t changed anything in the field catalog.
Figure 108 DOMNAME in Field catalog
