In previous versions of VHDL, it was illegal to use signals declared as out for anything other than an output.
So if you wanted to assign a value to an output, and also use that same signal for other logic, you would either have to declare a new signal and have that drive the output and the other logic, or switch from an out to a buffer type.
VHDL-2008 lets you use output values, as shown in the following example:
entity test is port(
in1 : in std_logic;
clk : in std_logic;
out1, out2 : out std_logic);
And then later in the architecture:
if clk’event an clk=’1’ then
out1 <= in1;
my_reg <= out1; -- THIS WOULD HAVE BEEN ILLEGAL in VHDL.
out2 <= my_reg;