Pairing the remote control (RC) to your Spark is normally a task for the first day of playing with your new toy: Do it with directions from the DJI GO 4 App or do it manually by pressing some buttons on the devices. But if you accidentally or on purpose un-pair them later on, you might get into trouble re-pairing them again. Just search on the Internet, it's full of such stories.
This step is essential for an accidental un-paring, e.g. by pressing the power button on the Spark for 6 sec until it beeps twice, which means “un-pair RC”, or even pressing it for 9 sec until it beeps thrice (yes, it's a word now), which means “wifi reset”.
The goal is to look for and write down the “11pkey”, which is essential for the link between this particular Spark and its RC. In a scenario where the parameters are reset on the air craft only, but not on the RC, you just have to restore said “11pkey” and set one other parameter to restore the pairing.
Now you've got your 11pkey.
Say, you've un-paired your Spark from the RC, accidentally or on purpose. And conventional means of re-pairing as described above won't work. What're you gonna do? Now the previously saved 11pkey comes in handy.
Of course, you can do the editing on an adb shell with busybox vi /data/misc/wifi/preference.conf, too.
Note: The 11pkey lives on the RC, too, of course. In a perfect world you would have to possibility to just look it up on the RC.
Note 2: Although there seems to be no indication on the Spark AC itself, it's possible that the RC has some kind of information with which AC it was paired beforehand (MAC wise), so it's unclear if this method can be used with a different Spark with a different Wifi MAC address.
Edit: Turns out, you can just align the MAC addresses of two Sparks by manipulating /amt/WIFI_nvram.txt on the AC to have two Sparks with one RC. But only one at a time, please.
Note 3: Pressing the power button on the Spark RC for 9-12 sec will reset it. From my experience this doesn't reset the 11pkey nor any possible AC MAC binding, just its Wifi AP's SSID and WPA credentials to the default values.