Because of their relative sizes and nearness to the earth, the sun and moon are not merely seen as points of light but as spheres with sizeable diameters.  Therefore, in order to take a sextant altitude, the observer needs to measure the angle between the horizon and their upper or lower rims.

This angle then needs to be corrected by either adding or subtracting their semi-diameters.  The point on the rim of the sun closest to the horizon is referred to as the sun’s lower limb and is denoted by either LL or O.The point on the rim furtherest from the horizon is referred to as the sun’s upper limb and is denoted by either UL or O.  If the sun’s lower limb is used then the semi-diameter correction is added to the apparent altitude.  If the upper limb is taken then it is subtracted.

Semi-diameter correction.