The Merge Rasters function represents a grouped or merged collection of rasters. Use it when you have multiple rasters that you want treated as a single item, such as calculating the same statistics for all, or when color balancing so you don't need to color balance each image separately. This is useful when working with imagery stored as separate tiles due to file size constraints; this way, the tiles will be treated as part of the same image.
This function can be inserted automatically for some imagery, but it cannot be directly inserted into the function chain.
To use this function, open the attribute table and select the rows in the table representing one image. Then right-click the Footprint layer and click Selection > Merge Selected Items. The selected items will be converted from individual items into one new item (row) in the mosaic dataset's attribute table. If you open the properties for this item (by clicking the arrow in the Raster field), you can view the Merge Rasters function in the function chain.
To remove this function, select the item in the attribute table, right-click the Footprint layer, and click Selection > Split Merged Items.