ELEMENT PLANE STRESS specification means that the plate can carry in plane stress only and no out of plane bending or shear. This is used for membrane type elements. IGNORE STIFFNESS on the other hand is assigned to plates whose stiffness would not be considered in the analysis. Only plate loads applied on such elements are considered and can be transferred to the supporting structural elements. This option is typically used to model glass facades of buildings or corrugated sheets for the roofing which needs to transfer wind or snow loads to the structure and does not contribute to the structural stiffness.
MEMBER TENSION on the other hand is an attribute for beam elements only and you do not need to assign these to plates.
MEMBER TENSION on the other hand is an attribute for beam elements only and you do not need to assign these to plates.