Let us consider an oligomer of composition 
which has no inter-chain connections.
This oligomer has six different chains (which I will call
A1, A2, A3, B1, B2, C for obvious reasons) of three different types
and will require six CHAIN statements. Because there are no links between
chains the CHAIN statements are very simple.
To complete the definition of this structure one would also include the RESIDUE statements defining the sequence for the ALPHA, BETA, and GAMMA chain types.
If we consider the Insulin tetramer the example becomes a little more complicated.
Here we have four A chains and four B chains. The A chains are linked to each corresponding B chain by two disulfide bonds (link type ``SS''). The first line reads: The chain named A1 is of type A and its residue 7 is linked the residue 7 of chain B1 by a SS bond and its residue 20 is linked to residue 19 of chain B1 by a SS bond. Each B chain has an internal disulfide bond as well. This link would be defined on the RESIDUE statements that define the chain type B.