What I call "line" is only confusing to you.
Take the X-Tn line: X-T1, X-T2, X-T3. It's a series of models in the same line.
The X-Pro-n line: X-Pro1, X-Pro2, X-Pro3.
The X-Tnn line: X-T10, X-T20, X-T30.
Etc.
Nothing confusing about this.
If you prefer, we could call a "line" a "series", but the meaning would remain the same.
Take the X-Tn line: X-T1, X-T2, X-T3. It's a series of models in the same line.
The X-Pro-n line: X-Pro1, X-Pro2, X-Pro3.
The X-Tnn line: X-T10, X-T20, X-T30.
Etc.
Nothing confusing about this.
If you prefer, we could call a "line" a "series", but the meaning would remain the same.