I mean that if I shoot one shell from a shotgun and then I press R, considering that I still have some shells left, the character plays an animation where he only reloads one shell in the shotgun. Adds a lot of immersion and just looks good.
Same thing with the mosing, the animation only shows that I (the character)put in only the missing bullets. I think that second or first CoD had this. I looks stupid that I reload five bullets even though I only shot one....
R is a general reloading animation this is context based on the weapon wielded. A weapon that has a magazine and has a spare one with some ammo in it changes the magazines. If the gun has no magazine but the character shot with it once, it will reload that bullet.
This is what will make a big difference between the Arma mod and the Standalone. I hated how there was one general animation for every gun which just looked awful. (especially pistols!)
Here are some examples:
http://www.youtube.com/watch?v=9uUQI5Qg4Uo
http://www.youtube.com/watch?feature=player_detailpage&v=yFy2YgQpXaY#t=109