本文部分内容源自xda论坛，渣翻译原创。
译文纯个人内容翻译，并非句句相对。
原文如下：
System read only option: Devices that ship with 5.0 and higher as their initial OS are using block level OTA updates. With this style of OTA update, the update script checks to see if the system partition has ever been mounted read/write. Further, the script also usually runs an SHA sum of the entire system partition to detect if any changes have been made. If any changes have been made, the OTA update will refuse to install. Since not all OEMs and devices have factory images available, we have created a new feature in TWRP that detects if the system partition has ever been mounted read/write. If not, you will be prompted asking if you want TWRP to mount system as read/write. If you choose not to allow TWRP to mount as read/write, TWRP won’t prompt to install SuperSU and TWRP won’t try to patch the stock ROM to prevent TWRP from being replaced by stock recovery. The goal of this option is to hopefully allow the user to make a raw system image backup that they can use to get back to a state where they can take OTA updates again.
[v_act]
译文：（这个是针对进入twrp后提示系统分区被修改，提示是保持只读还是修改分区的问题）
系统只读选项：
当系统版本是5.0或者更高的时候，zip格式的刷机包不再使用版本“解压zip包中的system文件夹到system”的方式
安装系统，而且使用“system.new.dat” 使用基于块的方式更新整个分区。当使用这个新式的方式ota安装系统时，
安装脚本不会去挂载system分区，而是通过sha算法校验整个分区是否被修改。如果被修改，那么更新将会中终止。
遗憾的说，这项新技术再国内未得到普及，所以不存在这个风险。直接选择修改系统分区好了。
resize2fs feature: On some devices like the Nexus 6, the factory images include a userdata image that is the proper size only for the 32GB units. If you flash the factory image to a 64GB Nexus 6, the data partition will appear as if it only has the free space of a 32GB device. Using the resize2fs option, TWRP can resize your data partition to take up the full space available. The resize2fs may also be useful to resize system partitions on devices where custom ROM system images don’t take up the full partition space. Lastly, resize2fs may be useful in some cases to reserve the proper space at the end of a data partition for a full disk encryption key, should your partition be formatted incorrectly for some reason.
译文：（这个是针对刷机后容量缩水的）
重定义分区大小功能：
有些设备比如nexus6，工厂镜像（一般指线刷包，TOT之类的东西），包含了userdata镜像（userdata.img）
刷入这个userdata.img会修改data分区为32GB，当你使用64G版的nexus6时，刷入后会显示只有32G的存储。
使用twrp的重新定义分区大小的功能，可以恢复原本的大小。
简单的说，线刷包中的userdata镜像的分区信息是32G的，你刷入64GB容量的手机中，那么自然就被修改成32G
但这只是标注为32G，可以直接改回来