Syntax question while dimensionning

I think you're needing to look into Geometric Dimensioning & Tolerancing. In the US the standard governing that is ASME/ANSI Y-14.5, which is on a par with ISO standards. You'll actually find it in section 5.13 and an illustration in figure 5-59, if you have the ability to look it up. I can furnish you with an example if you email me directly (remove/replace the capitalized MUNGE info in my email address), but the bottom line is that you establish the length of the bar as a datum by attaching a datum letter to the length dimension, then you use positional tolerancing to reference that datum (as the secondary or tertiary datum). The datum CAN be referenced at MMC (maximum material condition) or LMC (least material condition) which allow a bonus tolerance as the length departs towards the opposite end of the size tolerance. But in your case you probably want it at RFS (regardless of feature size) which gives no bonus tolerance at either end of the size tolerance. For that you DON'T need to add a symbol to the datum reference (the datum is simply presumed to apply at RFS).

Now, whether or not the machine shop will understand what you're actually specifying is a different story. Probably the best thing is to add a note to the drawing.

Mark 'Sporky' Staplet>

For the simple answer, a note that says "PART IS SYMMETRICAL."

WT

Good point. In that case the note could say "HOLES ARE SYMMETRICAL ABOUT THE CENTERLINE."

WT

But some time the part is not symetrical, just a few features.

message

I do not need GD&T, I just need to know if i should write on the dimension sometihing like relative to center or something like that.

Well, you asked for "the standard way to dimension so that it is relative to center" . . . and as far as I know there is no other STANDARD way.