Every section has a count which lets the engine know it's reading a new section,
The planetItemType Count is not just for bonuses, it holds everything that goes into templates, ships, structures, artifacts, bonuses. Take a look at a couple templates, everything in there uses the designName which it looks for in the planetItemType list and calls the linked entity file.
Also, its the set that is 1 count, not the amount of new lines, eg
adding those lines, you would increase the count by 1