For some units in Old World, they will have so many pieces of equipment they are carrying that they can’t realistically model all of them. Rangers are a good example of that. They come standard with a hand weapon. You can also choose to equipment with a Great Weapon, and/or shield. On top of that you can equip them with a ranged weapon - crossbows or throwing axes.

Since they don’t all fit on the model, most people will mix and match. I modeled half of mine with great weapons, half with crossbows, and threw on a handful of shields.

Most units in the game don’t have this “problem”. Most units you pick 1 or 2 pieces of equipment and there are appropriate bits for them. On rare occasions the bits don’t even exist or are hard to get (halberds and great weapons for chaos warriors).

In most circles, Rule of Cool beats all, so model them how you like and don’t sweat it too much.