Nodes come is a variety of different shapes. Irregular shapes have irregular bounds that define the shape's hot area. The hot area is important for several reasons:

Hot areas can be taken for granted with built-in symbols because they are pre-defined and entirely transparent to the user of the program. However, hot areas are particularly important with imported graphics. All imported graphics are rectangular even though they most commonly contain a background and irregularly shaped contents. Therefore, when they are imported, a hot area or fitted bounds is determined automatically based on the image. This feature makes imported graphics easy to use as symbols in your diagrams.