Traditional DC coupling is PV panels feeding a charge controller, which charges batteries.
The inverter and DC loads draw from the batteries.
In most systems, no communication occurs, and each just watches battery voltage. Maybe monitors cycles to do an equalization charge occasionally.
AC coupling means you have a battery inverter which serves as a UPS (if grid tied), charging battery from AC and producing power from battery when necessary. If you attach a grid-tied PV inverter (e.g. Sunny Island, Enphase, others) to output of the battery inverter, it converts PV to AC and with the right features it adjusts how much power it delivers.
When the UPS is on the grid, frequency is 60 Hz and GT inverter delivers full power to the grid.
When off-grid, the UPS ramps up frequency. With the default settings of my Sunny Boys, they deliver 100% up to 61 Hz, then linearly ramp down to 0% at 62 Hz. Whenever there is a load fluctuation (A/C kicks on or off), the UPS supplies/absorbs the difference momentarily, and readjusts frequency.
The key feature to make this work is "frequency-watts", which is now an optional feature of UL-1741SA compatible inverters.
This document lists which SMA Sunny Boys are compatible with Sunny Island for off-grid and for grid-backup systems.
https://files.sma.de/downloads/SB-OffGrid-TI-US-en-22.pdf
Sunny Island was probably the first battery inverter to do that, now several other brands too.
You would wire Sunny Boy into the breaker panel on output of the Sunny Islands. If you do not ever feed it from utility grid, configure Sunny Boy in off-grid mode (or maybe called "island". If it is sometimes on-grid, you're supposed to configure Sunny Boy for "Backup = On All" or similar, and install RS-485 modules and run cable from master Sunny Island to all Sunny Boys. Some models of Sunny Boy don't have functioning "backup" or don't have RS-485 option, so support told me to set Sunny Boy for "off grid". I would get such instructions from them in writing and keep a copy before doing that, because it goes against their published instructions.
If you do both AC and DC coupling, definitely should have one or the other schemes to inform Sunny Island about DC charging by Midnight.
If your loads drain battery so low (80% DoD) that Sunny Island shuts off, Midnight will eventually recharge battery to 50% where Sunny Island will restart. I don't have any DC coupled PV so that would never happen. Instead, I have a "load shed" relay which disconnects my house from the panel with Sunny Boys at 70% DoD. That leaves enough charge to keep Sunny Island up until the sun rises again. Even with Midnight, you can do a load-shed relay so your AC coupled PV contributes and charging is faster, don't have to wait for only the DC coupled PV to get battery high enough.
The nice things with AC coupling is the additional inverter capacity helps power loads during the day, and Sunny Island controls their output so battery charge current can be a constant optimum, rather than varying so much.
See diagram with Sunny Boys on output of Sunny Island, by house (load shed relay not shown.)
This diagram also shows some new communication paths supported by the latest model Sunny Boys. I have older models.
That communication is optional.