This is the part of the resourcepack that includes mostly textures that you may encounter most often. A lot of upper world stuff, literally.

This pack includes a color variation of some blocks, such as wool, concrete, ceramics or cement. White versions of these blocks are located in the BASE part of the resource pack.

This pack includes a lot of blocks that are either underground, or can be extracted from there, or are somehow indirectly connected to the dungeon. If necessary, you can replace this part of the pack with a lower resolution.

This pack includes all the vegetation that is in the game, such as standard foliage, grass or flowers. You can use this part of the pack in a lower resolution, if the detail is not important to you.

This pack only includes items.

This pack includes new models, such as new 3D foliage. If you don't want new models, just don't use this part of the pack
It is important to have Iris&Sodium or Optifine for this part.