The table reads:
Test piece diameter
D>25 (for test piece tube diameters greater than 25 mm)
Range of qualification
>=0.5 X D (25 mm min) - The range qualified is greater than or equal to 0.5 times the test piece diameter, but that the minimum diameter qualified is restricted to 25 mm. For diameters less than this you need to use a test piece with a diameter equal to or less than 25 mm.