As a complete novice with little understanding of LEGO terminology what does that phrase mean? If certain things are legal does that also mean that mean that other things are prohibited? What/who defines LEGO legality?
The question referenced in the comments doesn't answer my question because it doesn't define what "legality" actually is. The referenced question provides a few examples of "illegal" connections but because of my knowledge (or lack of it) I'm still unable to determine what actually is "legal".