boot options in /boot/loader/entries file

I'm working on configuring a GPU passthrough and would like to understand the options line in /boot/loader/entries/*conf.

options crashkernel=auto resume=/dev/mapper/rhvh-swap rd.lvm.lv=rhvh/rhvh-4.4.2.1-0.20200929.0+1 rd.lvm.lv=rhvh/swap root=/dev/rhvh/rhvh-4.4.2.1-0.20200929.0+1 boot=UUID=db556fa1-8850-4afb-a96d-6c8018d98610 rootflags=discard img.bootid=rhvh-4.4.2.1-0.20200929.0+1 null

rootflags is a mount option per the mount man page. What is the "null"? It is present on some of our systems and absent on others.