An antenna is classified as Dual Linear if it contains two internal elements operating as a single unit, in which the first and second elements radiate polarised waves whose electric field vectors are orthogonal to each other.
Single unit is an arbitrary term which may refer to any aspect of the antenna's operation, including but not limited to, operating frequency band, described application, or multiple element technology.
The classification is less restrictive than defining element polarisations, but more restrictive than 2X2 MIMO in that it requires orthogonality of polarisations.