Sinclair ZX81 User Manual page 76

Array of characters:
The part string at E_LINE contains the line being typed (as a command, a program line, or INPUT data)
& also some work space.
The calculator is the part of the BASIC system that deals with arithmetic, & the numbers on which it is
operating are held mostly in the calculator stack.
The spare part contains the space so far unused.
The machine stack is the stack used by the Z80 chip to hold return addresses & so on.
The GOSUB stack was mentioned in chapter 14.
The space for USR routines has to be set aside by you, using NEW as described in the last chapter.