6.C.7. & 6.C.8. Hand Signals to Indicate "In" Rulings

6.C.7. All “out” calls must be promptly signaled by voice and/or hand signal. "In rulings" may also be signaled by voice and/or hand signal.

6.C.8. In doubles play, if one player calls or otherwise indicates the ball “out” and the partner calls or otherwise indicates it is “in,” then doubt exists and the team’s call will be “in.” Any player may appeal the team’s call to the referee or defer a line call to their opponent up until the next serve occurs. If the referee or opponent did not see the ball, the ball is considered in. 

6.C.7. All “out” calls must be promptly signaled by voice and/or hand signal.

6.C.8. In doubles play, if one player calls the ball “out” and the partner calls it “in,” then doubt exists and the team’s call will be “in.” Any player may appeal the team’s call to the referee or defer a line call to their opponent up until the next serve occurs. If the referee or opponent did not see the ball, the ball is considered in.

It seems like a definition for "In Ruling" is prudent.

Per 6.C.7. (All “out” calls must be promptly signaled by voice and/or hand signal.) If a player’s hand signal is acceptable as an “out” call, a hand signal should also be acceptable for an “in ruling.”  We understand that the majority of "in rulings" by default do not require a verbal or non-verbal indication. However, in the situation where partners may be indicating conflicting line calls, there should be an allowance for both verbal and non-verbal indications for both "out" calls and "in" rulings.

This type of hand signal "in ruling" from a player's partner should be recognized as an expression of a different call, as described in 6.C.8. Hand signals should be equivalent to verbal expressions.

A player verbalizes an “out” call after the ball bounces on the court surface and appears to land past the team’s baseline; his partner immediately indicates by hand signal that the ball was “in.”