DI stands for Direct input or Direct Inject and is really just an audio signal. It is used to convert or transfer one signal type to another. Often a guitar unbalanced signal to a microphone balanced signal. Some more elaborate DI boxes provide the ability of Ground Lift, which means it uses a different signal for the ground of the input signal and the ground of the mixer, this sometimes reduces mains interference but it is dependent on where the interference originates. DI output, especially on Accusound Amplifiers, is a signal that is a copy of the main audio signal within the amplifier, that can be used to input to another mixer or amplification system that the user may require of recording or providing a signal to another location or part of a location.
In addition, Accusound amplifiers have a Pre/Post switch associated with this DI output signal. Pre and Post refers to the location in the audio chain from which the output signal is taken. Pre refers to before the main output volume control and Post refers to after the main output volume control. By doing this a signal can be output that is not affected by the output volume of the initial amplifier, this is PRE as the signal is before the output volume control and in effect outputs a signal as if the volume control was at maximum. This is good for relaying the signal to another system, recording or amplification and allows the controller of that system to have maximum control over the level they use.
When used as Post, the signal sent to the auxiliary systems is controlled in level by the output of the main amplifier. This can be good if the signal is sent to an extension loudspeaker or an annex room. In these cases, the level in these rooms will be increased and decreased in line with the adjustment of the main volume on the main amplifier.