External CLK in Artix-7
You can drive a clock into any CC (clock-capable) pin and use it as a GCLK or feed to a PLL or MMCM block.

There are special uses for some clocks, for example to implement memory interfaces, that have local low-skew local distribution in addition to driving a global net. That’s where MRCC and SRCC come in. MRCC inputs are Multi-region. SRCC are single-region.

The diagrams in the doc you linked explains SRCC and MRCC in more detail: https://www.xilinx.com/support/documentation/user_guides/ug472_7Series_Clocking.pdf#G4.319704