Outside to inside
A structure such as PET 12 / Met PET 12 / LDPE 50 lists each film from the outside of the pack to the side that touches the product, with its thickness in microns. Ink and adhesive sit between the films but are usually left out of the name.
Each layer is chosen for one job. Reading a structure is mostly a matter of knowing which job each film does.
The print layer
The outer film carries the artwork. PET and BOPP are the usual choices. The design is normally reverse printed on the inside face of this film, so the ink ends up sealed between layers where it can't scuff or touch the product.
PET is stiff, clear, and stands up to sealing heat. BOPP costs less and is a good moisture barrier, which is why it is common on snack packs.
The barrier layer
The middle layer, when there is one, keeps out what spoils the product: oxygen, moisture, light, or aroma loss. Metallised PET or BOPP gives a good barrier at moderate cost; aluminium foil gives the strongest barrier; BOPA (nylon) adds puncture resistance for sharp or vacuum-packed products.
The sealant layer
The inner film melts and bonds to itself when the packing machine seals the pack. LDPE is the common choice: tough, seals over a wide temperature range, and safe for food contact. CPP is used where the pack needs more heat resistance or clarity.
The sealant is usually the thickest layer, because it gives the pack most of its strength.
2, 3, or 4 layers
Two layers (print film + sealant) suit products that only need moisture protection and a short shelf life. Three layers add a barrier film for longer shelf life. Four layers add strength or a second barrier, for heavy, sharp, or export packs. More layers mean more cost, so the right structure is the simplest one that protects the product for its shelf life.