ABM Lite runs campaigns against small clusters of accounts that share an industry, use case or buying situation, typically ten to fifty at a time. It sits between Strategic ABM and Programmatic ABM in ITSMA's taxonomy, trading some personalisation depth for the ability to cover more accounts with the same team.
A shared problem, not a shared attribute. Grouping by employee count gives you accounts with nothing to say to each other. Grouping by a situation, such as recently acquired, or replatforming, or newly regulated, gives you a message that is specific without being bespoke.
Because the economics work. One piece of creative covers twenty accounts rather than one, while still being specific enough that a reader recognises themselves. Most programmes that claim to run strategic ABM at scale are in fact running ABM Lite well, which is a better outcome than running strategic ABM badly.
They are the same thing under different names. ITSMA's taxonomy calls it ABM Lite, most vendors say 1:few. If a vendor uses one and you use the other, you are talking about the same tier.