I think:
1. Alternate RAM is for ST, it's above 4 MB, and below of 16 MB. Max size is 8 MB, so in total 4MB ST + 8 MB Alternate RAM = 12 MB in total. Alternate RAM is not accessible for shifter, blitter and DMA controller and STE-DMA-Sound as these chipsets only use 22 adress lines (2^22=4MB. Processor has not to share Alternate RAM with these chips, so he can access it without a handbrake. So it's a kind of fast-RAM. Alternate RAM needs driver because TOS is not designed to detect it by itself. The space between 12 and 16 MB in adress room is reserved for hardware registers of the chipsets and TOS ROM.
2. Falcon RAM, is almost the same as ST+ALT-RAM, it's also below of 16 MB, but it is in one piece as Falcon TOS can detect it without driver help. It's fully accessible for CPU, Videl, SCSI, DSP, blitter, DMA-Sound, ... They have to share it's speed, so if for example Videl does some demanding graphics modes (high res plus many colors plus high refresh rate), this slows down CPU and all other. To be exact, ST-RAM in Falcon is always 16 MB, but the upper 2 MB are hidden and not accessible. Otherwise there would not be any adresses left to adress the hardware chipsets, VME-Bus (in Mega STE) and TOS ROM inside the 16 MB adress room.
3. ST-RAM in TT can be 2 MB onboard + 2, 4, 8 MB ST-RAM add on card. All of the ST RAM in TT, so up to 10 MB, is accessible by CPU, TT-Shifter, DMA-Sound, DMA-Controller and SCSI chip. Speed is shared between them. TT-TOS automatically detects complete ST-RAM up to 10 MB, no driver is needed. Above 10 MB the adress space is reserved for hardware registers in chipset, VME bus and TOS ROM.
4. TT RAM in TT is above 16 MB, depending on TT-RAM card it could be up to 256 MB (and maybe more, 32 bit adress bus means an adress range of 4 GB), TT-TOS can autodetect it. It's only accessible for the CPU, the other TT chipse can not access it. That's fast-RAM for TT. (TT mirrors M-STE compatible VME adress space also to above 16 MB, so the full adress room is not available for memory)
5. TT-RAM in Falcon is not possible without turbo card. This is because Falcon mainboard is 16 Bit data and 24 Bit adress design, that means, Falcon can only adress 16 MB in 16 Bit mode. Falcon has 68030 CPU which is basically capable to adress 32 Bit adresses and 32 Bit data, but mainboard design is not that good as it could be. It has been made to be cheaper. TT-RAM in Falcon is also above 16 MB but it needs replacement CPU in the system bus socket. That turbo card can have internal 32 Bit data and 32 Bit adress architecture, but the connector to the Falcon is 16/24 Bit, so it can access all Falcon mainboard hardware and RAM (and other peripherials) and above 16 MB in 32 Bit mode. That RAM above 16 MB is also fast RAM as it can not be accessed by Videl, DMA-Sound, DSP, SCSI-Controller, IDE-Controller, blitter and so on. CPU has it for it's own. However as Falcon is only 24 Bit adress bus design, Falcon TOS still is capable to detect TT-RAM above 16 MB automatically if it is present together with a 32 Bit adress mode CPU (68030-060) on the turbo card.