It is where the bearing is on the shaft. A top bearing is between the blade and the shaft that connects to the router. A bottom bearing is at the end opposite the shaft and router. As the bearing is the part that runs along the template or guide, the placement of the bearing determines how the router is used and where you have your template. Generally speaking, a flush trim bit is considered to be a bottom-bearing bit. However, top-bearing bits, also known as pattern bits, can be used equally as flush trim bits. As we see from number 2 in our reviews, top and bottom bearing bits are also available. The preference is where the template will be placed and whether you use a handheld or table router. For example, with a handheld router, most find it easier to have the template on top of the wood, so a top-bearing bit is needed to follow the template while the blade cuts the wood below. Conversely, if you use a table router with a template on top of the wood, you would use a bottom bearing bit because the router bit sticks up from the table. An advantage of a top-bearing bit is that it can also be used for cutting grooves and holes because the cutting blade is left exposed at the end. This would not be possible with a bottom-bearing bit as the end is covered with the bearing.