The "holding capacity" listed for each clamp refers to the maximum external force that clamp can safely resist in its locked position without incurring any permanent deformation. For hold-down clamps, holding capacity is measured with the spindle as close as possible to the handle, and will be less at the end of the clamping arm.
Clamping force, the amount of force actually applied to the workpiece by closing and locking the clamp, is generally less than the stated holding capacity. This allows the clamp to have ample capacity to resist external machining forces. Actual clamping force depends on many factors including: spindle position, proper spindle-height adjustment, spindle cushioning (uncushioned hex-head spindles provide the greatest force), workpiece material, and the clamp's mechanical advantage. In most cases, roughly 1/2 of the stated holding capacity can be applied by hand with medium effort.