Well, it depends on your bit and the design; what we mean is that the bottom flat of your bit is, literally, the finest detail of your carving possible. Think of it as the ‘resolution’ of your output. In other words, if your bit had a “tip” that was 1/32" of an inch wide, then you cannot create any detail smaller than 1/32" wide (technically you could with elaborate milling paths, in some instances, by basically carving away the outside of a feature, but I’m talking about carving out material).
So if your bit as a 0.1mm flat, then you cannot get a feature carved out that’s wider than 0.1mm. Easel doesn’t acknowledge a flat on your v-bit, so you have to program a custom bit for that to have Easel render it properly. Remember, Easel trusts that the bit you used in Easel is the same as your actual bit; it doesn’t do error-checking because it can’t sense that, so you have to be sure the geometry of your tool matches the geometry of your software model.