A distance interaction record has three fields separated by colons ":". The first field is the type I or D already described. The next two fields are spock selection strings identifying the atoms that are involved in the interaction. When an interaction file is read, if there is more than one atom that matches the given selection, the first atom in the sequence is used.
An intensity interaction record is nearly identical except that the first character is I and there are two extra fields (colon-delimited again) for the interaction type number and the intensity of the interaction.
Here's a sample interactions file:
! interaction file written by spock v1.0, Copyright 1997, Jon A. Christopher I : r=ALA,rn= 1,ch=_,a= CB : r=HOH,rn= 174,ch=_,a= O : 1 : 1.000000 I : r=SER,rn= 35,ch=_,a= OG : r=HOH,rn= 196,ch=_,a= O : 5 : 2.000000 D : r=LYS,rn= 41,ch=_,a= NZ : r=PRO,rn= 55,ch=_,a= CD D : r=ASP,rn= 66,ch=_,a= OD1 : r=HOH,rn= 155,ch=_,a= O I : r=HOH,rn= 124,ch=_,a= O : r=HOH,rn= 192,ch=_,a= O : 3 : 0.000000 D : r=HOH,rn= 181,ch=_,a= O : r=SER,rn= 64,ch=_,a= O I : r=HOH,rn= 192,ch=_,a= O : r=HOH,rn= 193,ch=_,a= O : 2 : 0.000000