The SUMSQ function syntax has the following arguments: Number1, number2, ... Number1 is required, subsequent numbers are optional. 1 to 255 arguments for which you want the sum of the squares. You can also use a single array or a reference to an array instead of arguments separated by commas.
THEY/THEIR (SINGULAR). Using the plural pronoun to refer to a single person of unspecified gender is an old and honorable pattern in English, not a newfangled bit of degeneracy or a politically correct plot to avoid sexism (though it often serves the latter purpose).
It is a typical linguistic term and originates from grammar. Often antecedents and their respective pronouns agree in number, which means if an antecedent is singular, the pronoun that replaces it will also be singular. However, sometimes writers might not follow this rule, and we see singular antecedents are replaced with plural pronouns.