The start/stop feature in the current BMW 3 Series is, by default, activated every time you get in and start the car. If you don't like it, you can deactivate the feature by pressing a button on the dash; however, the next time you turn the car off and start it again (with the key), the start/stop system will kick in.

This is too much hassle for some drivers, who wish start/stop to remain off unless they decide otherwise. To accommodate them, BMW has given dealers permission to activate the system's Last User Mode, which will start the car with stop/start in the same mode as when the car was shut down. Of note, BMW M models come with Last User Mode enabled automatically. There's a service bulletin below with the particulars.
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INFORMATION
The Automatic Start/Stop (MSA) system is a further BMW Efficient Dynamics measure aimed at meeting BMW's commitment to reducing CO2 emissions and enhancing fuel economy.

BMW, always a leader in innovative technology, will be the first automobile manufacturer to bring MSA technology to all new models.

Automatic Start/Stop systems will become prevalent in the future, as manufacturers strive to meet increasingly stringent emissions and fuel economy requirements.

By automatically switching the engine off when the car is stationary, MSA can improve fuel economy and reduce emissions.

New F-vehicles now incorporate the second generation of MSA (MSA II), which has been further developed to operate in combination with an automatic transmission and the BN2020 electrical system.

A detailed description of the MSA Automatic Start/Stop function can be found in the ICP Technician library under Technical Training course ST1112.

SITUATION
Some drivers have expressed the desire to deactivate this function in certain situations. This can be done manually via the Automatic Start/Stop function button with the LED switched on, indicating that the start/stop function is deactivated.

However, by default, the Automatic Start/Stop function is reactivated each time the engine is started.

At the request of an owner, it is possible to modify this default logic to "Last user mode."

With this logic, the current MSA's activated or deactivated setting is stored and used on the next trip.

Note: All new BMW M models have the "Last user mode" as the default setting from production.