Terminology as used in the project.
The Platform Firmware is a generic term used to describe the first thing
the CPU starts at boot time. On your typical
x86_64 system, it would be what
was previously called the BIOS. Now often diminutively called by the name
EFI. This is what initializes enough of the hardware so that the operating
system can start. Additionally, it often provides facilities for the user to
do basic configuration, and manage boot options.
In the ARM with SBCs landscape, U-Boot is the de facto solution for the Platform Firmware. Though U-Boot is confusingly, but rightly, often referred to as a Boot Loader. U-Boot plays double duties often. It is tasked with initializing the hardware, and often also used to handle loading and booting the operating system.
The Boot loader is a program which may or may not be distinct from the Platform Firmware, which is meant to load and boot an Operating System.
Boot loaders are now commonly written following the UEFI spec. GRUB, rEFInd and systemd-boot are examples of UEFI boot loaders.
Dedicated storage is used when the Tow-Boot installation lives on a location where it is not expected the target booted system would live.
Generally speaking, an SPI flash chip is dedicated storage.
A built-in eMMC, even if small, would be considered shared storage. Note that the eMMC may have hardware boot partitions, these are considered dedicated storage, but may not be usable on all platforms.
Shared storage is used when the Tow-Boot installation lives on the same storage as the booted system.
While technically some say it wouldn't be shared storage, if the storage where Tow-Boot is installed is also a bootable target, it is shared storage. Even if the actually booted system lives entirely on another storage.
E.g. on a Raspberry Pi, reserving the whole SD card for the FAT32 partition for the Raspberry Pi firmware files and for the Tow-Boot install, and booting a GPT-formatted USB drive, Tow-Boot is considered to be installed as shared storage.