The Mean Motion Double Dot is defined as one sixth the second time derivative of the Mean Motion, measured in orbits per day per day per day (orbits/day³). It basically defines the rate of change of the Mean Motion Dot from day to day so that some orbit propagation software can continue to predict the location of the satellite accurately.

The mean motion double dot is normally zero unless the satellite is being maneuvered or is undergoing orbit decay.

The format of the Mean Double Dot is:

Sign: (nil or -): The sign of the digits succeeding it. No sign indicates positive;
First five digits: A number with a decimal point assumed to precede the first digit;
Sign (nil or -): The sign of the exponent. No sign indicates positive; and
Last digit: The exponent (base 10).

If a Mean Motion Dot Term was "-12345-6" within a TLE, it would mean -0.12345x10^-6 orbits/day³ or in scientific notation:

-1.2345x10^-7 orbits/day³.

Not all orbit propagators read or use this value.