An AMD boot image format (BIF) file is used to specify the subsystem images in the PDI and the subsystem image partitions. Each separate file within a subsystem image is stored in one of the subsystem image partitions. The BIF file is a data file in ASCII format. The BIF file tells Bootgen how to create the PDI by processing each of the input files.
If required, you can modify the Vivado-generated BIF and run Bootgen using the command line flow to create an updated PDI. You can optionally write your own BIF file and specify the Vivado generated PDI along with other input files to extend the PDI. In the Vitis IDE, you can use a wizard to specify the required inputs for the BIF file, and then use the Vitis IDE to create the BIF file and run Bootgen to create the PDI.