Be another/a different matter
Definition 1 of 2
Pro Tip 1/3Separate Concern
Use the phrase to emphasize issues are unrelated and should be considered independently. 
She passed the written test, but the practical exam is a different matter.
Pro Tip 2/3Conversation Shift
Indicate a shift in conversation to a new topic that needs separate attention. 
As for our sales figures, they're good. Customer satisfaction, though, is a different matter.
Pro Tip 3/3Contrasting Importance
Suggest that the second issue has a different level of significance or urgency. 
He's on time today, but his overall attendance is another matter.