Would you like to be able to make identical curves or shaped details on more than one piece of stock? Then you need a flush-trim or a pattern bit. With the bearing at the top (the end opposite the bit’s shank), it’s called a flush-trim bit and, as the name implies, trims one surface flush with another. The most obvious use is trimming applied laminates level with the edge of a countertop — that one function makes a flush-trim bit the most common bit used with laminate trimmers — but it can really make just about any two surfaces flush. The bit does this with cutting edges that are exactly even with the guide bearing. As the bearing rides one surface, those cutting edges level the adjacent surface with the first.
Move that bearing to the bit’s shank, and it becomes a pattern bit. It functions in the same way with cutting edges that are even with the bearing’s rim, but in this case the bearing generally follows a removable template affixed to the top of a workpiece rather than a permanent surface. By rough-cutting the workpiece slightly larger than the template, the bit will turn out multiple finished pieces, each matching the template exactly. All you need to do is temporarily attach the template, use it to guide the pattern bit, then remove the template and attach it to the next workpiece.
While they can be used in table-mounted applications, flush-trim and pattern bits are most typically used in a handheld router.