Transmission coefficient S21

Hello my question about Transmission coefficient S21, I researched in several books and articles that it transmits in -3dB, but does not inform the following condition. When lower value better transmission? Example: better [-60dB] than [-3dB]. All material uses -3dB value, which I understand in theory that in S11, when it is lower, the better the reflection coefficient, the lower the signal return at the source. Example: better [-30dB] than [-10dB]. What I understood in the transmission when smaller, the lower the insertion loss, transmits better.

I need guidance, please.

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Consider attenuator.
Its S11 is very small, however S21 is also small.

Not sure to have correctly understood you question.
However let's consider to have a two port system. Connect now a generator to port 1 and a load to port 2. You will have part of the power sent to the load and some back to the generator. The power going to the load will be the input power attenuated by S21, the power coming back will be the input power attenuated by S11. The same if the generator is applied to port 2 and the load to port 1: in this case the parameters will be S12 and S22 with the same meaning as before.

A quite rough answer to you question is :
If the network is passive S21 will be always negative. In this case if you want to send as much power as possible to the load S21 has to be as high as possible, that is -2 dB is better than -10 dB. If, instead you want to attenuate the power toward the load, S21 must have the level of attenuation you want.
If the network is active, then S21 can also be positive. this happens in amplifier, where S21 correspond to the gain.

In both passive or active networks S11 should be as small as possible (i.e. -30dB, -40dB, etc.) if you want a good matching with the generator impedance. S11 is a negative number in passive network and should also be a negative number in active network to ensure stability.