DC offset is when the waveform is not properly centered on the line, something which may occur if there is something introducing DC (direct current) into the signal. If that were to be reproduced all the way through to the speakers it be harmful or at least waste headroom. Most gear doesn't pass DC, but you never know.

Reversing (inverting) polarity is simply flipping the waveform upside down. Positive and negative are swapped. Sometimes when you have multiple microphones, such as on a drum kit, inverting polarity on one or more (but not all) can alter the effect of a sound getting into more than one mic. Polarity is often and wrongly called phase.

So when should i be using DC offset ?

K-Ro, post: 346086 wrote: So when should i be using DC offset ?

Actually, not at all. DC offset is not wanted in audio signals. It is often introduced by faulty equipment and/or earthing. You try to avoid it and in all DAWs there are features for removing the DC offset.
So, in DAW land it is less of a problem. In analog you must get rid of the defective part in the chain...
In most cases removing the offset is freeing your soundpicture a bit. Some smaller amounts of DC level is not killing anybody, though, but I do get rid of it as a standart procedure.
Do that before you start mixing or mastering. Removing it later might change the sound and causes extra work to regain the sound you already had.
In analog, one aspect of the DC offset can be explaned like this: it is like connecting a little battery to the speaker cable leads. It causes the coil of the speaker to push or pull the cone in one direction by a small amount.
That hinders the cone to swing freely when the AC of the audio signal is exciting the coil. There are other electrical negative influences, like impedance matching, which might get us too deep into this matter.

Servus, Big K

Thanks, i see now (y)