LOL - that's a simple question with a very technical answer!
Technically (genetically) speaking, there is no such thing as semi-flashy. Only classic (plain), flashy and ultra-flashy (white). This is because the difference between flashy or not is actually genetic. The "semi-flashy" category is a useful description for a dog whose genetic makeup can't be determined for certain just by looking at it. BUT even if we call the dog "semi-flashy" it *will* be genetically either flashy or classic. There is no in-between
Take a look at our coat colour genetics page here: www.boxerworld.com/coat_colour
That gives some basic information about the coat colour combinations it is possible to get, and also of flash.
In very basic terms though, there are two genes governing pigment distribution in boxer dogs. There is S - a gene that produces mostly
solid coat colour (except for a bit of white usually confined to the chest, toes, tip of tail) and there is Sw - the extreme white spotting allele, which produces lack of pigmentation (white).
All genes come in pairs - one from each parent. So if you get a dog that has two copies of S then you get a classic/plain boxer. If you get a dog that has two copies of Sw, then you get a white boxer. And if you get a dog that has one of each gene, then you get a flashy boxer - essentially the effects of both genes combined.
Just to complicate it further, these genes are affected by modifiers. This is why all flashy boxers do not have identical markings, or why some white boxers have patches of colour while others don't. It is also why there are some boxers who have an amount of white that we commonly describe as "semi-flashy". The dog is genetically either
a plain individual (two S genes) where the unpigmented areas, affected by modifiers, are more widespread than one would normally expect; or else it is a flashy individual (One S gene, one Sw gene) whose unpigmented areas, again affected by modifiers, are not as widespread as you would normally expect.
Does that make sense to you? There are quite a few threads on the forums discussing the genetics of flashy vs plain that go into more detail, if you care to read.