CHAINS is used with PDBOUT.
Protein chains are normally labeled A, then B, then C, etc. If this sequence is correct, then keyword CHAINS is not needed. If a different sequence is to be used, put that sequence in place of text in CHAINS(text).
For example, in PDB entry 2V66, the chains are B, C, D, and E, not A, B, C, D; for this system, the keyword would be: CHAINS(BCDE).
CHAINS is also useful if there is a break in the reported sequence. Thus if a protein consists of one chain with three breaks where residues are missing, then, in order for the PDB file to have the correct chain-letters, keyword CHAINS(AAAA) would be necessary
See also START_RES(text) to define residue numbers.