Basically, anyone who launched a public campaign qualifies. If you're just short selling without any related posts about why you're short, etc. you're not an activist short seller.
It's ambiguous, no doubt, but actually pretty easy to discern in practice. There are a few big names that dominate the landscape, as well (e.g. Citron).
It's ambiguous, no doubt, but actually pretty easy to discern in practice. There are a few big names that dominate the landscape, as well (e.g. Citron).