A theoretically exact dimension, shown boxed, with no tolerance of its own.
A basic dimension has no tolerance because the tolerance is supplied by a feature control frame. It locates a feature exactly, and the allowable variation comes from the GD&T callout attached to it.
In a CAD model with basic dimensions, the model is the authority: the drawing's basic dimensions are exact, and the 3D geometry must agree with them.
Tolerancing a basic dimension. It carries none by definition — adding one contradicts the feature control frame that is meant to govern it.
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