The memory chips used in both the ECU and ignition controller are what's known as volatile memory. Any stored diagnostic codes as well as the dynamic fuel trim program data (from the last 16? starts) are lost. Rather than trying to remember the push button sequence to reset any OBD codes and make the Check Engine light go out, many simply disconnect the battery for a moment. The radio may then lose its pre-sets or now need an unlock code, so pulling a fuse instead of disconnecting the battery is better. Either way the adaptive fuel trim program will need to relearn the normal running conditions, which is of no great real concern. I sometimes purposely do that to see if the engine now seems to run a bit better under default running conditions, suggesting that something is slowly getting out of whack, like an aging O2 sensor. Thinking of the Check Engine light (CEL), although the presence of most stored OBD codes will cause the light to go on, there are a few less important ones that don't (as often noted in tables), which is why it's best to always check for codes during diagnosis.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now