Heavy lines are drawn corresponding to XTickValues and YTickValues, provided that axis XGrid = 'on' and YGrid = 'on'. The positions do not need to be regular.
Lighter dashed lines are drawn corresponding to the axis XRuler.MinorTickValues and YRuler.MinorTIckValues, provided that XMinorGrid = 'on' and YMinorGrid = 'on' . However, I cannot find any way to customize the minor grid at the moment. I thought I had found a customization if XRuler.MinorTick is 'on' and YRuler.MinorTick is 'on' by adjusting XRuler.MinorTickChild but that turns out to only affect drawing ticks at the MinorTickValues location. I guess in theory you could adjust XRuler.MinorTickChild.VertexData and related parameters to extend those tick marks across the axes, but I don't know that it would be worth the trouble.