That image is intended to describe REIL. REIL are are two synchronized flashing lights. They can be either omnidirectional or unidirectional facing the approach area. Unidirectional REIL are preferred. More information is found in AC 150/5340-30.
Installation requirements are in section 7.7.5:
Locate and aim the REIL units per Figure A-79.
188.8.131.52.1 When possible, install the two light units equidistant from the runway centerline.
184.108.40.206.2 When location adjustments are necessary, the difference in the distance of the two lights to the runway centerline may not exceed
10 ft (3 m).
220.127.116.11.3 Each light unit must be a minimum of 40 ft (12 m) from the edge of taxiways and other runways.
18.104.22.168.4 The elevation of both units must be within 3 ft (0.9 m) of a horizontal plane through the runway centerline, with the maximum
height above ground limited to 3 ft (0.9 m) (See Figure A-93.)
22.214.171.124.5 When the centerline elevation varies, use the centerline point in line with the two units to measure the centerline elevation.
126.96.36.199.6 Orient the beam axis of an un-baffled unit 15 degrees outward from a line parallel to the runway and inclined at an angle 10 degrees above the horizontal.
188.8.131.52.7 If this standard setting is operationally objectionable, provide optical baffles (per the manufacturer's instructions) and orient the beam axis of the unit 10 degrees outward from a line parallel to the runway centerline and inclined at an angle of 3 degrees above the horizontal.
As described here, the 15 degrees is the horizontal alignment, and the 10 degrees is the elevation alignment.