An ICP combines company criteria and person criteria. Company criteria include industry, size, geography, revenue and business model. Person criteria include role, seniority, department and whether you can reach them at all.
The test of an ICP is whether it changes a search. "Mid-market SaaS" does not. "Software companies with 50 to 500 employees where a marketing director owns demand generation" does.
In Buzzlist the ICP lives in your Hive, which Buzz drafts from your website and you confirm. Search, briefs and email writing all read from it.