Weight Painting
Weight painting is a technique used in rigging to assign influence to specific parts of a 3D model by painting weights onto the vertices of the model. These weights determine how much each bone in the rig affects the deformation of the model at any given vertex. In other words, weight painting defines how much a bone's movement will influence the shape of the mesh at different points.
Weight Transfer
Ragnar Hrodgarson:
When it is a simple Armor, for example only cloth or leather and if it is plate not much overlapping parts you could just transfer weights and then make small fixes here and there, mostly in the armpits, or depending if there are belts or other stuff in the hip area.
For every Armor that is more complex, that could potentially be used as base to start with, but sometimes it is just better and faster to make the weightpaint completely yourself then.