A grouping that has been created in the current or a previous session of CellexalVR is stored in a .txt file. All selections are saved in separate folders under Output/<username>/<dataset>/Selections. The selection.txt has the following format:

Where each cell_1 cell_2 and so on are the names of the cells that were selected, color is the color that the cell was given, graph is the graph it was selected in and group is an integer that is unique to the group.
If you are creating this file on your own and not using one that CellexalVR created for you and do not care which graph the cells come from; you do not have to fill in the graph column. Just type anything (like "unknown") and CellexalVR will choose a graph for you. Make sure you type in something though, as empty entries are not allowed.

Along with this text file is a number of images, each group in the selection and each graph that was loaded by CellexalVR when the selection was saved will generate one image, you can read up more about how CellexalVR use these images in the programmer's guide. In short, they contain information used by CellexalVR to much more quickly re-colour graphs based on some pre-defined colouring, like a selection. Each pixel in the images represents a cell in CellexalVR's graphs, and the pixel's colour defines which group they are in. The image that ends with _combined contains all of the groups combined into one image.