A conjunction used to add a proviso, exception, or condition to what was just stated (often written-only/formal).
参加は無料です。ただし、事前登録が必要です。
Participation is free. However, prior registration is required.
商品は返品できます。ただし未使用に限ります。
Products can be returned, provided they are unused.
明日は出張します。ただし午後は時間があります。
I will be on a business trip tomorrow, but I have time in the afternoon.