Rule 6.6.2.1

Here is a complete answer (with input from members of both the BANA Tactile Graphics committee and the NBA Mathematics committee):

I assume that the "Rule 6.6.2.1" you're asking about is from the Guidelines and Standards for Tactile Graphics. Further, I think what you are asking about is the fourth bullet under 6.6.2.2, which says
[quote=]Axis lines with arrows indicating the indefinite nature of the axes of the graph should be positioned so that the arrowheads are outside the boundary of the grid. If space does not permit the placement of the arrowheads outside the grid, the tips of the arrowheads may be positioned to reach the outer-most grid lines.[/quote]
In this guideline, "indefinite" is almost the same as "infinite." So, if an arrowhead at the end of an axis indicates that the axis goes on forever, then the arrowhead "indicates the indefinite [or infinite/unending] nature" of the axis. This tends to be the set-up of most grids, and the arrow at the end of an axis indicates that the axis goes on forever, whether the arrowhead is inside or outside of the grid.

For all practical purposes, any arrow on an axis line is infinite, whether placed inside or outside the grid lines.

So, a definite axis line will have no arrowhead while an indefinite one will.

Does that clarify things for you?

Thank you for your question and your attention.
–Kyle