I found this symbol on the schematics of an FPGA board. AIN stand for Analog input.
If you didn't cut off the picture, you'd see the wires are labelled
AIN16_N. The red symbol tells Altium that these signals (each matching
N pair) should be considered a differential pair.
This can allow the layout tool to invoke special design rules for these nets, and the autorouter or interactive router to treat them specially.