true center is halfway down the ski, perfectly in the middle. Core center is the middle of the core, which is like 4cm back from true center probably. 'Recommended' is usually like 6 or 7cm back from true center and is useful mostly for mountain skiing.
How much park/mountian/BC and stuff do you ski? that'll help, then we can recommend where to mount your skis. Basically, forward from recommended or center mounting is better for switch skiing and park, and back is better for mountain and powder all that.
Of course some people will disagree but thats generally how it goes.