A member is a structural element designed to carry loads between or beyond points of support. The orientation of the member (being horizontal, vertical or sloped) is not relevant to its definition (contrary to a beam or column). A member represents a linear structural element from an architectural or structural modeling point of view and shall be used if it cannot be expressed more specifically as either a beam or a column. It can also simply be an architectural element which is non-load bearing.
A curtain wall is an exterior wall system of a building which is an assembly of components, hung from the edge of the floor/roof structure rather than bearing on a floor. A curtain wall mullion is a linear member within a curtain wall system to connect two (or more) panels.