NoPP: Independent pronouns that can replace the subject do not exist.
PPDrop: The presence of an independent pronoun in subject position is not required to replace a nominal agent.
NonPPDrop: The presence of an independent pronoun in subject position is required to replace a nominal agent.
 A personal pronoun is considered independent if it can be used without a verb – for example, in response to questions like Who is the oldest? Who’s there? Who can do it? – or if it appears in a sentence with a verb but is not immediately next to it. Even if multiple personal pronouns appear in reference to the agent, only one is independent, and only it is considered in terms of this parameter.
 Independent personal pronouns should not be confused with other, unstressed dependent words that are in fact elements of verbal conjugation. The latter may be required while independent personal pronouns are not.
 This means that a conjugated verb can form a complete sentence alone. Dependent personal pronouns (such as je in je pars ‘I am leaving’ in French) are considered to be part of the verb, and so even languages that require them are considered to be of the PPDrop type.