Don't know what type of Nortel it is by your post. If it's a PBX (61C/81C) it is defined according to set type.
Digital sets (2000 series)it is defined when conf. key is added to key in LD 11 as follows:
Key (number) AO6. For analog sets it is defined in LD 10 in CLS as follows:
cls C6A (for 6 party conference allowed).
Don't know if it relates, but thought I'd try and help.