Why is separation in TLC described as surface-based rather than bulk-based?

Points on why TLC is described as surface based:

• Thin layer chromatography uses a shallow adsorbent layer on a plate to create surface-based separation rather than bulk separation within a medium.
• The separation process remains limited to the exposed adsorbent surface instead of occurring throughout a packed or deep material.
• Because the adsorbent depth is minimal, separation takes place along the plate surface during solvent movement.

To learn more about it check out definition of thin layer chromatography for more information.