One rather inconspicuous detail I like very much about the classic #travellerrpg is the notion of setting up custom tasks. While there are only a limited amount of default actions given, those serve as examples for an unlimited amount of possible actions.
For example: To pick a mechanical lock roll 9+, DEX 10+ and each level in mechanics adds +1 to the roll.
Once such task has been defined it becomes a fixed rule in the campaign.
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