Simple question about transistor datasheet Опции

[Sara00]

hello i see the 2n3553 NPN power transistor for rf.
i see these statements

Minimum Gain: 10dB
Efficiency: 50%

what is "Gain: 10dB" meaning?
why they show us the effeciency of transistor?it doesn't related to circuit that we design? for example class A or class C?

Thanks