An operational assumption is a formal or stated assumption you base your design or operating procedures around.
For example, in your contract with customer X, you stated that the system will never have more than 1 million records and that your team is not responsible if more than 1 million records are added by the customer.
It may be true that the customer eventually puts in more than 1 million, but your OperationalAssumption is that your system will not handle that condition and you don't design for that condition. (Although it's probably a good practice to at least display an error message such as "Too many data items have be entered" or the like to reduce confusion for both parties if and when it does happen.)
An actual example is that Intel does not design its off-the-shelf chips for use in outer-space or a nuclear battle zone. There's too much radiation to flip bits in those conditions. The OperationalAssumption is that they will be used on the surface of Earth in a peaceful area and thus they don't put space-friendly safeguards into them. If a space company puts one in a space probe and it goes kaflooey; well, they operated outside of Intel's OperationalAssumption. (There are companies who specialize in radiation-hardened chips.)