some questions about the linkcmds file

[Poletti Francesco]

Hy,
In the linkcmds of the armulator there's:
/*
 * Declare some sizes.
 */
_RamBase = DEFINED(_RamBase) ? _RamBase : 0x0;
_RamSize = DEFINED(_RamSize) ? _RamSize : 1M;
_HeapSize = DEFINED(_HeapSize) ? _HeapSize : 0x10000;
_StackSize = DEFINED(_StackSize) ? _StackSize : 0x1000;

but in the how to bsp manual I See something like this:
RamSize = DEFINED(RamSize) ? RamSize : 1M;
HeapSize = DEFINED(HeapSize) ? HeapSize : 0x10000;
StackSize = DEFINED(StackSize) ? StackSize : 0x1000;

Which difference there about this two tipe of scpripture? In particular what 
between  _RamSize and RamSize and what is _Rambase?
Why in the Vegaplus's linkcmds file there isn't the definitions above HeapSize 
and StackSize?

Thank to all Francesco