Full Idea
Three degress of law: 1) 'Oaken laws' where all Fs that aren't Hs are Gs; 2) 'Iron' laws where all Fs are Gs; and 3) 'Steel' laws where all Fs must be Gs.
Gist of Idea
Oaken conditional laws, Iron universal laws, and Steel necessary laws
Source
report of David M. Armstrong (What is a Law of Nature? [1983], 10.4) by PG - Db (ideas)
Book Reference
Armstrong,D.M.: 'What is a Law of Nature?' [CUP 1985], p.148
A Reaction
[My summary of Armstrong's distinction] One response is to say that all laws are actually Oaken - see Mumfor and Mumford/Lill Anjum. It's all ceteris paribus.