For some applications, a data-acquisition card with a multiplexer won't suffice. In automotive crash testing, for example, you need to make synchronized measurements, often from several accelerometers. There's a propagation delay between the time the master card samples and the time the master's sample clock reaches the slave card or cards. The slaves will be in sync with each other, but they will sample slightly later than the master. To compensate, a master card might store the measurements of the master in memory for one or two ADC clock cycles.
Some manufacturers fix the delay time based on their card's performance.
Others let you program this delay through a card's driver. PXI cards and VXI cards, though, extend their PCI and VME buses, respectively, by adding trigger and clock lines to their bus backplane. These additional lines were originally added to the VME bus, which became the VXI bus. The PXI and VXI bus add a trigger bus and a star trigger to the CompactPCI bus and the VME bus for synchronizing measurements across cards. You can set up your PXI and VXI cards to make any of the eight lines on the trigger bus as a trigger output, trigger input, clock output, or clock input.