A sideline is an alternative to the main continuation you prepared. A transposition reaches a familiar position through a different sequence. Both are easier to handle when your repertoire is organised around positions and ideas. If you only remember a chain of move names, a harmless change in order can feel like a completely new opening.
The diagram can arise after 1.Nf3 d5 2.d4 Nf6 3.c4 e6 or after 1.d4 d5 2.c4 e6 3.Nf3 Nf6. White has the same pieces and pawns on the same squares and it is White's turn. The same strategic knowledge applies. For exact position identity, also consider castling rights and en passant possibilities, which are not always obvious from a diagram alone.
Do not assume that every similar setup transposes. An extra pawn move can weaken a square, and a delayed knight move can permit an option that was previously unavailable. Compare what the opponent could do at each branch. This is particularly important when combining Reti, English and Queen's Gambit lines in one White repertoire.
Cover sidelines by frequency and consequence. Learn a safe, understandable answer to common alternatives, then investigate sharp rare lines when they appear in your games. Record where a sideline returns to known territory. That reduces duplicated study and helps you focus on the genuinely different decisions. In practice, stop at the unfamiliar position and explain what changed before choosing a move.
Try this at the board
Replay the two move orders in this lesson and identify the point at which they first become identical.