Let's take for example an MSX computer with 64K RAM. That's what you get on boot:
Where does this 28815 come from?
To start with, a MSX is a Z80 based machine. The Z80 has an addressing space of 64K. The MSX standard divides these 64K in four 16K pages that can be independently switched to any internal or external memory slot.
When booting in BASIC mode (the default unless there's a disk system present or a game cartridge inserted) the first page is switched to the BIOS ROM and the second page is switched to the BASIC interpreter ROM. This leaves two more pages that are switched to RAM, so 32K RAM are available.
Now, the MSX system reserves some RAM for BIOS work area. This area starts at F380h, so this turns those 32K into 29568 bytes. Add (or better, substract) one extra K or so for BASIC-specific work area and the stack itself, and that's it, 28K free.
(Side note: when a disk system is present additional work area is required for MSX-DOS, so the free space in BASIC turns into 23K)
And what happens with those "invisible" 32K RAM in the lower two pages?
Nothing. That is, BASIC does nothing with them. The MSX2 standard introduced a limited RAM disk mechanism that made use of that memory area, but other than that, that RAM was "wasted" as far as MSX-BASIC was concerned.
Of course you were free to use that extra RAM from BASIC using the BIOS inter-slot read/write routines (you'd need custom assembly code for that, though) but from the point of view of the BASIC interpreter itself that RAM didn't exist.