The difference between a crack and a separation is real but not always appreciated. Separations are where two materials that are joined become separated. A crack is where a material fractures, especially due to being overloaded. Here are some examples.
Examples of cracks
Cracks are unusual. Most people would not recognize a crack even if it was shown to them. Rafter cracks
Examples of separations
Caulk separating from a metallic window, wood joints in trim joints that were initially tight. separations between bricks and the mortar, metal and plastic corner beads separating from the stud framing,
If a separation is the same width from top to bottom, then the separation is not likely to be due to foundation distortion.
Separations are common. They occur due to the fact that different materials react differently to changes in temperature and humidity. They do not tell us how much the foundation has deflected.
Cracks are less common and can, in some circumstances, be used to estimate deflection. Cracks can be caused by temperature and moisture changes. Cracks are mostly confined to curing cracks in concrete, cracks in floor tile and cracks in drywall.