If the LabelUp value of a Plot axis is non-zero, it causes numerical labels for that axis to be plotted upright (i.e. as normal, horizontal text), otherwise labels are drawn parallel to the axis to which they apply.
The default is to produce upright labels if the labels are placed around the edge of the plot, and to produce labels that follow the axes if the labels are placed within the interior of the plot (see attribute LabellingLabelling).