I would think it refers to the pump being sealed to deter tampering.
All meters in gas pumps are sealed with a crimped seal, to help protect consumers from getting less volume than is actually displayed on the register.
If a seal is broken, it would be caught on next pump inspection/calibration and reported. Unscrupulous Station operators would do this to increase their profits.
These days...I believe Shell puts tamper stickers on the cardreader access doors at the pumps, to help prevent credit card fraud.