Many guitar amps feature a Presence control. Depending on the specific amp, the Presence is generally operates as a high treble control, adjusting the range around 4kHz or so. The control is so named because when you boost the amp at this frequency, the result is that the tone tends to cut through, almost like it's being highlighted. In other words, it is more present.
Live sound mixers can boost this frequency using the mixing board EQ to bring out nearly any instrument or vocal in a mix that needs emphasis. Just a bit of a boost is generally enough — don't go crazy.
Do you want to emphasize the lead singer? Boost her around 4kHz. Does the guitar solo need to cut through? Boost it around 4kHz. Are the drummer's cymbal hits getting lost behind the guitars? You guessed it: boost around the presence range of 4kHz.
Of course, this isn't a magical cure-all. In fact, if you don't use this type of EQ very carefully, you'll end up with too many things boosted at 4kHz, and a harsh strident mix. But if you use this type of EQ very selectively, you'll be able to use it to emphasize an instrument when you need to have it pop out of the mix.
The assistant to the main recording engineer that is running the session. An experienced assistant engineer will be the one to place microphones, stands, cables, music stands, and other items in preparation for the session. He or she may operate tape machines, break down equipment after the session, and perhaps most important, document the session — in other words, the assistant takes care of all the ancillary audio recording tasks, allowing the main engineer to focus on what is being recorded or mixed.