An applier is a scripted hud, that applies a texture to a mesh body part. For example, if you wear a mesh breast such as Lolas :::Tango::: you cannot wear a system top because that would leave your breast naked. You would need an ‘applier’ to apply the matching texture to your boobs. The same goes for a mesh avatar like the ToddleeDoo Child avatars. They need an applier to be able to wear clothes made from clothing textures/templates. The huds are usually provided by the creator of the mesh body part/avatar, but there are also universal appliers such as Omega, that will fit a number of mesh body parts/avatars.