Emergency Boot is a setup with default parameters.
If you select Emergency Boot, the Secondstage Loader looks for a bootable system in the flash memory and then resumes the boot procedure as in the other boot modes.
Essentially speaking, the X-Server is started without network drivers and with a resolution of 1024 x 768 - 60 Hz during an Emergency Boot. The Setup menu is then opened directly.
This option is useful if, for example, you have selected an excessively high screen resolution or a wrong mouse type and these settings can no longer be changed in the normal setup.