“chain” selects atoms based on their assigned chain, which is a one-letter code.

chain chain-identifier-pattern
c. chain-identifier-pattern

indicate chain A
indicate c. A:C             # selects chains A, B, and C

A blank or undefined chain identifier is entered as two apostrophes.

chain ''

The macro equivalent is chain-identifier-pattern//.

indicate A//
indicate A+C-G+X//

bychain

Selections | Selection Macros | Chain Concept