Pixel type is another way of saying LED pixel chip type. There are a variety of LED manufacturers who use various different pixel chips. The pixel type is identified by a series of letters and numbers, like WS2801 and APA102. Pixel LED chips vary in color depth, dimming smoothness, number of pixels that can be used in a series, the number of wires used to control them, and many other characteristics.
When working with RC4 dimmers, pixel types are identified and selected by their alphanumeric identifier. The most common type is WS2812 by World Semi. Some pixel chip types are compatible with others. For example, APA104 pixels can be controlled using the WS2812 setting in RC4 pixel drivers.