What do animation numbers mean and how can I control which number is used?

[heavemnoon0107]

Hi everyone,
I'm creating a new handheld item for my mod and I'm really confused about the animation numbers.
Why are the image numbers different when I decompile the official animations?
For example:

walking_stick only has 0
glasscutter has 0, 2, 5
spear has 0, 5, 6, 7, 8

I can display a specific image by writing "BUILD" in the animation, but it only shows that one image. Why are there so many images in the official animations if I can only display one?
The glasscutter shows -0 when held normally, and -2 when placed on the ground for gathering twigs.
What do these numbers mean and how can I control which number is used?