Series A, B, and C are funding rounds that generally follow “seed funding” and “angel investing,” providing outside investors the opportunity to invest cash in a growing company in exchange for equity or partial ownership. Series A, B, and C funding rounds are each separate fund-raising occurrences. Series A is the first major round of business financing by experienced SPV syndicators and venture capitalists. An “A” round by external investors generally takes place after the founders have used their seed money to provide a “proof of concept” demonstrating that their business concept is viable – and could eventually be profitable. SPVs are used to participate in all funding rounds, Seed, A, B, C and beyond.
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https://www.investopedia.com/articles/personal-finance/102015/series-b-c-funding-what-it-all-means-and-how-it-works.asp