On power-up the DEL key will take you into the BIOS or the F7 key can be used to go straight to a boot menu.
The ViewSonic SC-T46 BIOS identifies itself as Aptio Setup Utility and is copyright AMI. My unit was running version SCT46R07. Scanning the Linux dmesg output I see: DMI: ViewSonic Corp. SC-T46/t180, BIOS SCT46R07 10/01/2014
It supports both Legacy and UEFI booting.
Essentially what appears in the boot options depends on what the BIOS finds as the system powers up.
With a 4GB pen drive plugged in and various legacy options selected the F7 key brings up:
Please select boot device:Realtek PXE B01 D00 UEFI OS (PO: SATA SSD) PO: SATA SSD UEFI: Built-in EFI Shell UEFI: SanDisk Cruzer Blade 1.26 Enter Setup↑ and ↓ to move selection ENTER to select boot device ESC to boot using defaults
In the BIOS 'Boot' screen you get an identical list under 'Boot Option Priorities'.
My experience here is that there aren't any - which is a pity.
On the ViewSonics.com website the only downloads I could find were for the SC-T46 datasheet. I had previously found the page https://www.viewsonic.com/uk/products/archive/. That is still there but the SC-T45 and SC-T46 are no longer listed on that page.