In allen bradley processors, SQO is SeQuencer Output. It is an indexed masked move with a built in counter. Each time an SQO instruction goes false to true, it increments it's position value and moves the indexed source through an indexed mask to an output word. When the position value is greater than the length value, the position value resets back to 1. The position value is only zero when forced to zero programmatically or when the PLC processor first enters run mode.
So, they're used to control a machine which has a defined list of steps that must be completed in a sequence. Each step has certain outputs turned on and others turned off. These on and off states are contained in the source words that are used by the sequencer instructions.
To get more than 16 bits in your SQO, program them in parallel and use the same control address for each.
Hope this helps...