Addgene diagnostic digest
This gel depicts the NEB 1kb ladder and a digestion of the plasmid with the following enzymes: XbaI; SalI; XbaI+SalI double digest.
XbaI was most likely destroyed during cloning. Hence XbaI+SalI double digest gives the same band pattern as SalI alone. The bands that can be seen with SalI are - 2.1kb, 2.5kb and 4kb. The band sizes add up to the length of the full plasmid.