What does 挂号 (guà hào) mean in English?

Card 358HSK 2010 Level 5358/1311
CARD358
PROGRESS27%
挂号
掛挂
guà hào
to register (a letter etc)
Click to reveal
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Reveal: Click/Space/Enter/↑↓/w/s/j/k•Navigate: ←→/h/l/a/d