You wil find books on the same subject will have the same number. The shelfmark is preceded by the letter D and will be followed by 3 letters, which are usually the first 3 letters of the author's name.
Here are some useful physics shelfmarks.
D 530 Comprehensive works on Physics
D 531 Mechanics of solids
D 532 Mechanics of fluids
D 533 Mechanics of gases
D 534 Sound and related vibrations
D 535 Optics
D 536 Heat
D 537 Electricity and electronics
D 537.622 Semiconductors
D 538 Magnetism
D 539 Molecular, atomic and nuclear physics