Under the hood translate5 uses Okapi for file conversion and segmentation.
The segmentation rules can be customized by customizing the SRX rules file, that is packaged inside the Okapi bconf file, that is used inside of translate5 to configure Okapi.
When importing a zip import package into translate5, you optionally can add a bconf-file (batch configuration file) from Okapi Rainbow to it on the root level of the zip file. Then translate5 will use this bconf file for parsing your imported documents instead of the default one.
The batch configuration file is a bconf-file as exported from Okapi Rainbow.
For customizing SRX rules we recommend to start with the okapi_default_import.bconf, that you find in your translate5 installation at /application/modules/editor/Plugins/Okapi/data.