The UCL or upper control limit and LCL or lower control limit are limits set by your process based on the actual amount of variation of your process.
The USL or upper specification limit and LSL or lower specification limit are limits set by your customers requirements. This is the variation that they will accept from your process.
Below is a control chart illustrating this. The specification limits (customer related variation) will always be wider then the control limits (process related variation).
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