If companies are related, then they are deemed to deal not at arm's length. “Arm's length” is broader than “related”, because if you're not related, you can also be considered not to deal at arm's length as a result of other factors. If you're related, you're not at arm's length, but you could also be not at arm's length even if you're not related.
On May 25th, 2023. See this statement in context.