To solve problems it is important that the dimensions of the answer are those that are input to the problem. In Engineering the dimensions of an item are important so that it withstands the loads it is subject to.
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Datum dimensioning is where a single reference point is used for all dimensions in a single direction. E.g. If Point A is declared the reference point, then Points B, C, D, etc, will all be measured relative to Point A, not to each other. Chain dimensioning is where each point is measured to the next. E.g. Point B is measured from Point A, Point C is measured from Point B, Point C is measured from Point B, and so on. The problem with chain dimensioning is that each measurement relies on the accuracy of the previous measurement. If there is a tolerance associated with each measurement (+/- X), then the tolerance for each subsequent dimension accumulates and becomes the number of measurements times the tolerance of each measurement (+/- n*X). Since datum dimensioning uses only one reference point, the tolerances cannot accumulate as each measurement is isolated, rather than relying on the accuracy of the previous measurement. Common practice is to only use datum dimensioning in design drawings to ensure greater accuracy over the entire design without requiring tighter tolerances.
Multi dimensions Or more easy using both metric and inches to represent dimensions
It is called postion and it is represented as a circle with a plus sign in it. For the symbol representations GOOGLE: Geometric Tolerancing Symols.