The attached file is an excerpt from the JY997D16601 manual from Mitsubishi. A common question asked about these PLCs are which devices are battery backed, how do I set that up and configure it etc.
With these PLCs there are a pre-defined set of battery backed registers and it changes for each model, FX3S, FX3G and FX3U are all different.
Some example details:
Input and Output quantities for each series are shown below, as are M bits, D registers, and T timers. For more information see the attached PDF file or the full manual.