NbtNb (Non-binary-to-Non-binary), sometimes refered to as XtX, is a term for transgender people who consider themselves transitioning from non-binary to non-binary, as they has always been non-binary, just not in the way being non-binary is traditionally defined as. They have always been non-binary and always will be non-binary, regardless of whatever genital, hormonal, or chromosomal arrangements they were born with, and whether they chooses to make changes to their body or not.
It is an alterative to FtNb and MtNb, as FtNb and MtNb imply that one was male or female at some point, then transitioned to non-binary, which does not accurately describe how some non-binary people view their gender. Female-to-Non-binary and Male-to-Non-binary are framings that are easier to explain, but only because they're still rooted in the concept of binary sex distinctions and assuming gender based on sex, which many trans people want to do away with entirely.