Q: Can I use another iMac as a display on a late-2015 27" iMac?

1. Yes.

2. If you’ll be using Target Display mode, a Thunderbolt cable. If you’ll be using Virtual Network Computing, any network connection.

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Target Display mode uses a video connection and is faster, but requires that the computer being used as a display has specific hardware, and isn’t usable on nearly as many computers.

Virtual Network Computing sends the data over an existing network connection.

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VNC allows you to see and control the other computer. It does NOT allow the other computer to be a secondary screen for the original computer.

So if you want to use an iMac as a secondary monitor for your 2015 iMac, that other iMac has to meet the criteria defined in Use your iMac as a display with Target Display Mode - Apple Support