RE meaning in Space Science ?

Earth radius (denoted by the symbol R🜨 or by R E {\displaystyle R_{E}} ) is the distance from the center of Earth to a point on or near its surface. Approximating the figure of Earth by an Earth spheroid, the radius ranges from a maximum of nearly 6,378 km (3,963 mi) (equatorial radius, denoted a) to a minimum of nearly 6,357 km (3,950 mi) (polar radius, denoted b).

A nominal Earth radius is sometimes used as a unit of measurement in astronomy and geophysics, which is recommended by the International Astronomical Union to be the equatorial value.

A globally-average value is usually considered to be 6,371 kilometres (3,959 mi) with a 0.3% variability (+/- 10 km) for the following reasons.The International Union of Geodesy and Geophysics (IUGG) provides three reference values: the mean radius (R1) of three radii measured at two equator points and a pole; the authalic radius, which is the radius of a sphere with the same surface area (R2); and the volumetric radius, which is the radius of a sphere having the same volume as the ellipsoid (R3). All three values are about 6,371 kilometres (3,959 mi).

Other ways to define and measure the Earth radius involve the radius of curvature. A few definitions yield values outside the range between polar radius and equatorial radius because they include local or geoidal topography or because they depend on abstract geometrical considerations.