Set-off comes in three main categories: legal, equitable and contractual. 'Legal' is the simplest, being a deduction of an existing debt against a newly-arising one. Because equitable set-off is the more complex of them it results in more cases.
An attractive feature about set-off is that it opens the potential to assert an old debt against the plaintiff as generally it is exempt from limitations, if it is advanced as a defence - and not a claim.
Set-off - Legal
Set-off - Equitable
Set-off - Contractual
Set-off - Limitations
Note: The Courts of Justice Act [CJA s.111] has provisions allowing for set-off of mutual debts.
[last edit 07 Nov 2022]