Absolute allows you to define an address in memory where you want to have the first byte of the variable. Syntax:
[pascal]
var
Variable: <type> absolute segmentffset;
[/pascal]
Register allows you to register the convention call to define the parameters of the procedures and functions. The first three parameters of functions or procedures bearing the directive will be available via the processor registers, in turn EAX, EDX, ECX, and other parameters will go to the stack in the order from left to right. Example:
[pascal]
procedure Foo(A, B, C, D, E: Integer); register;
begin

end;
[/pascal]
In this procedure, the value of A parameter will be in the register EAX, B - EDX, C - ECX, D and E (in that order) will go to the stack.