Draw Reference Planes

The Draw menu > Draw Reference Planes command establishes a point that is then projected as a horizontal plane.

Reference planes are horizontal planes at user-specified Z-ordinates. The main purpose of those planes is to provide a horizontal plane/line that can be used to snap to when drawing objects in elevation views. Reference planes also can be viewed in a plan view. This option can be useful for adding mezzanine-type framing when the mezzanine has not been specified as a story level in the story data.

Note the following about reference planes:

When vertical frame or shell objects (columns or walls) are drawn in plan on a reference plane level, ETABS inserts one object from the reference plane to the story level below and another object from the reference plane to the story level above.
When vertical frame or shell objects (columns or walls) are drawn in plan on a story level and there is a reference plane in that story level, ETABS does not break the vertical object at the reference plane. A single object is drawn from the story level to the story level below.

Draw reference planes as follows:

Click the Draw menu > Draw Reference Planes command. The Properties of Object form will display in the lower left-hand corner of the ETABS window.
On the Properties of Object form, enter a value in the Vertical Offset Z edit box to specify the location of the reference plane.
If more than one coordinate system has been defined in the model, the Coordinate System drop-down list can be used to select the coordinate system for the reference plane.
Left click at a point in an Elevation View to draw a horizontal reference plane.
Exit the draw reference plane command by clicking the Select Object button or by depressing the Esc key on the keyboard.