  In approval-based committee (ABC) elections, the goal is to select a
fixed-size subset of the candidates, a so-called committee, based on the
voters' approval ballots over the candidates. One of the most popular classes
of ABC voting rules are ABC scoring rules, which have recently been
characterized by Lackner and Skowron (2021). However, this characterization
relies on a model where the output is a ranking of committees instead of a set
of winning committees and no full characterization of ABC scoring rules exists
in the latter standard setting. We address this issue by characterizing two
important subclasses of ABC scoring rules in the standard ABC election model,
thereby both extending the result of Lackner and Skowron (2021) to the standard
setting and refining it to subclasses. In more detail, by relying on a
consistency axiom for variable electorates, we characterize (i) the prominent
class of Thiele rules and (ii) a new class of ABC voting rules called ballot
size weighted approval voting. Based on these theorems, we also infer
characterizations of three well-known ABC voting rules, namely multi-winner
approval voting, proportional approval voting, and satisfaction approval
voting.
