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Only five words hold R second and B third, and each one takes a different opening vowel.
O opens two of the five, with A, E and U taking one apiece. Four letters appear in that box, so the first letter almost identifies the word on its own.
That is as informative as an opening box gets. Only ORBED and ORBIT share a starting letter, and they part company in the fourth.
Both are ordinary English and a puzzle will nearly always want one of them. ARBOR, the American spelling of arbour, is recognisable but far less likely.
ERBIA is an oxide of the rare earth element erbium, and ORBED simply means shaped like an orb. Neither is vocabulary you would reach for.
Both are in everyday use. The opening letter separates them completely, so a guess carrying O or U in the first box settles the page.
ARBOR is the American spelling and arbour the British one. Only the shorter form fits five letters.
Erbium oxide, a pink compound used in glass and ceramics. The element was named after Ytterby in Sweden.
RB is a consonant pair English uses inside longer words like HARBOUR and SUBURB. At five letters only a leading vowel makes it work.