You should not flex your boards. A cracked solder joint on a rigid part is possible. Looking for rigid parts with the along axis aligned across the bow of the flex.

The flexing creates shear forces within the board, depending upon your via count and size and amount of plating, you can easily shear your vias and even tear them loose from the traces.

If you are looking at the different parts of the PCB, such as a motherboard. Denote, where the 4.3 volts, 5 volts and other voltages come in, these are known as rails on the motherboard, as in power supply rails.

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