When you are copy-pasting onto the same character, activating the Gravity option ensures that the feet stay at the original level of the pasted pose, keeping a natural orientation of the feet on the floor.

When the Translation (T), Rotation (R), and Gravity options are active, pasting a pose onto a character matches the translation on the global X- and Z-axes, and rotation on the global Y-axis. The original global Y translation value and the original global X and Z orientation values are preserved.

Gravity works best when you are copying and pasting poses between the same character. Pasting poses between characters of different sizes may produce undesirable results, causing the feet of the target character to lift into the air to match the pose of the larger character. For best results, disable Gravity when pasting poses between different sized characters.

You can also disable the Gravity option if you are pasting poses in a situation where you want the character’s feet to remain in the air. When Gravity is disabled, you can achieve a perfect match without respect to the floor.