While most startups issue convertible notes or safes in seed financings, some issue preferred stock (which is standard for post-seed financings). Financings where the startup sells preferred stock are known as equity financings, since preferred stock is a form of equity.
Typically, startups create a new series of preferred stock for each equity financing. Startups issuing preferred stock in a seed financings will usually call the new series Series Seed or Series AA. These financings often use forms based on those used in post-seed financings, but that are specifically adapted for seed financings.
By convention, for post-seed investments, the series are designated by letters in alphabetical order. For example, the series created for the first post-seed financing is typically called Series A Preferred Stock. The series created for the next financing is usually called Series B Preferred Stock, and so on.