3#, 4#, 5# - these refer to the zipper size, measured by the width of the zipper when closed. Simply put: the larger the number, the thicker the zipper. Usually, the zippers on jackets are size 5#. Sizes like 8# and 10# are considered special zippers, very rugged and require custom orders, so they're rarely used. 4YG specifically refers to zippers for pants, which are 4# zippers with a YG slider. This type of slider has a lock, making it especially secure for jeans and casual pants, and they usually have metal teeth. The vast majority of branded clothing will have custom zipper pulls made for their zippers. These pulls often come in various shapes and are engraved with the brand's LOGO. It's a very detailed touch that, when done well, can become a distinguishing mark.