Regarding Dhamma, shouldn't we discuss and talk about the aggregates (khandha), sense bases (āyatana), elements (dhātu), noble truths (sacca), and dependent origination (paṭiccasamuppāda) to understand them? That's Dhamma teaching during training.
I've heard from the old days, before my time as a monk, there were English schools. Have you heard of English Speaking class? There, people of different nationalities weren't allowed to speak their native languages - only English during training. Similarly, during Dhamma training, we should discuss Dhamma.
Ordinary speech involves talking about buying, selling, trading, and attachment - these are connected with defilements (kilesa). That's what we mean by mundane talk. But shouldn't we study matters that transcend defilements? That's what we should discuss and contemplate.
Ordinary speech means worldly talk and stories that give rise to defilements. Don't we need to make this distinction? Shouldn't we discuss both wholesome and unwholesome actions? Aren't we taught to discourage harmful actions and encourage beneficial ones?
If we don't guide someone in this way, they might create unwholesome kamma. We must give proper guidance. Shouldn't we point out what leads to wholesomeness? Yes, make this distinction and study it.
Just now, didn't we discuss from killing (pāṇātipāta) to intoxicants (surāmeriya)? Isn't it taught that violating these creates unwholesome kamma? This is what we mean by 'restraint of speech.'
Don't do things that lead to unwholesomeness - that's restraint of speech. Don't speak words that lead to unwholesomeness. Isn't it taught that avoiding unwholesome actions is wholesome? That's how it is. Think about it - you might have been seeing it differently."