I have never seen the abbreviation LR5CIB before. What does it stand for?
LR5CIB.Ircat That is the file name and extension generated by Lightroom for photo file management.
When I upgraded to LR6, it created a new directory entitled LR5CIB-2.Ircat.
Can I just delete the LR5CIB.Ircat file from my computer?
You are the person who has to be 100% sure that you don't need one of these files. I would keep them both for a while.
After LR6 migrated your catalog to the new LR 6 format, you how have two sets of files and folders:
LR5CIB-2.Lrcat - your LR 6 catalog file
LR5CIB-2.Lrdata - folder containing your preview files LR 6
LR5CIB.Lrcat - your LR 5 catalog file
LR5CIB.Lrdata - folder containing your preview files for LR 5
if you are sure that LR 6 is working correctly, then you can uninstall LR 5 and delete its version of the catalog file and preview folder.
You can also rename the Lr5cib-2 files and folders as long as you keep the matching names.