Ian here with a quick article to explain how we go about rating stuff in our articles here on the JustPlay blog. You'll note that in this explanation I refer to "things" a lot, and that's because we try to keep a consistent rating system whether we're rating units, weapons, psychic powers, or something else.
Remember though, that all ratings are subjective and different players will always have different views. Our views are not necessarily correct, but should work as a good barometer for anyone unsure of a certain thing or how effective it might be. Ratings could vary if you have a very specific meta and ratings will vary as changes to the game happen via FAQs and such.
Most importantly, remember that "rating" something assumes some level of competitive play; if you love a model, or some fluff, then go right ahead and ignore ratings altogether!
So with that said, here's what we consider when we rate things:
S-Tier things are those units, weapons, or abilities that are truly excellent. Reasons tend to vary - factors such as being undercosted or aggressively costed, very powerful, supremely flexible, or a combination of things are often what makes something S-Tier. You'll see S-Tier things in most competitive lists and often in multiples, and they'll be used very often.
A-Tier things are solid staples that excel at one or more things, tend to be very aggressively costed, and very useful. A-Tier units will generally form the core of any good list, A-Tier strategems will be used often, and so on. Most competitive armies will use things of this quality as a minimum for the great majority of their choices.
B-Tier things are reasonably solid, not quite as powerful as higher tiers but often fulfilling a crucial purpose not otherwise covered by S or A tier things. Units may be included to gain access to a particular strategem, to enable a certain type of combo, or simply to fill a gap. Strategems might be used occasionally when the board state is right for them.
C-Tier things are generally not good enough to be used in competitive games but are still playable and usable despite that. They will generally offer nothing that another, better thing cannot offer but may occasionally show up in very niche roles.
D-Tier things are generally considered unplayable; they contribute very little to your army, are overcosted, or have other serious flaws. Units will rarely be seen outside of playing for fun. Psychic powers may be mostly pointless. Warlord traits might just be downright bad.
Want to see how we apply these tiers to things that we review? Then go check out the Warhammer 40,000
section of our site for loads of awesome 40k content!