If they give the cultivar name you know exactly which one you're getting, but if it's just the grex name it could be anything from that grex. For example, lilliputiana x Break of Day could be either Janet Sue or Gene McKenzie or any one of many unnamed/unregistered seedlings from that cross. Some grexes can vary quite a bit, particularly if both parents are complex hybrids, so you're taking a chance if you haven't actually seen the plant.

That doesn't mean that it's wrong to use the formula, it's still correct. Nurseries may list it that way for several reasons. It may be a cultivar that was never named, or they got the plant before it was named and they aren't aware that it now has a name. I bought a clone of Hannibal Lector x Foster's Pink Tip that is neither High Voltage nor Metis (the 2 named cultivars). Maybe it will eventually get a name, but the problem seems to be that it looks so close to High Voltage that giving it a separate name would be confusing, so for now it remains unnamed. It's a beautiful plant, not inferior in any way to the named cvs, but unless I had seen it with my own eyes I wouldn't have known that. If you order High Voltage, however, you know you're getting a good cultivar.

Thank you very much lisaclv. This's usefulness for everybody. I would like to share this lesson for another BROM. planter in Thailand. Please permit me.

Yes, go ahead, I'm glad it helped!