An index file is a file that incorporates a listing of the recordsdata in a listing, together with details about every file, similar to its dimension, date, and time of creation. In Photoshop, index recordsdata are used to retailer details about the layers in a PSD file. If you open a PSD file, Photoshop will routinely create an index file for that file. The index file is saved in the identical folder because the PSD file and has the identical identify because the PSD file, however with a .index extension.
Index recordsdata are necessary as a result of they permit Photoshop to rapidly entry details about the layers in a PSD file. This data is used to show the layers within the Layers panel and to carry out operations on the layers, similar to shifting, resizing, and deleting them. With out an index file, Photoshop must scan your entire PSD file each time it wanted to entry details about a layer, which might be a lot slower.