aggregate

When raster data (grid) is displayed on a map it resides in a special group called an aggregate, and its name is preceded by the string AGG_. An aggregate is another layer that contains grids and images.

Aggregates are special because you cannot add or delete elements from them but you can manipulate them. For example, you can modify the colour display interactively using the Colour Tool and add shadows interactively using the Interactive Shading tool.