For residential use, Square D makes loadcenters (breaker panels) for QO series breakers and HOM series breakers (they are not interchangeable).
When using a plugin-style breaker (QO and HOM both are) as backfeed breaker then a retaining kit
is required to hold the breaker in place (prevents the breaker from popping out of the panel bus). This also applies to most other brands of residential breakers.
A retaining kit is not needed for bolt-on breakers (such as Square D QOB breakers) which are more common in commercial/industrial applications.
Edit: fixed spaces in URL--
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