"=" or ":=" ?

Hello,

I'm trying to optimize assign_vertex_id() function provided by
pgRouting/PostGIS. In this function, I can see :

DECLARE
points record;
i record;
source_id int;
target_id int;
pre varchar;
post varchar;

srid integer;

BEGIN

BEGIN
DROP TABLE vertices_tmp;
EXCEPTION
WHEN UNDEFINED_TABLE THEN
END;

EXECUTE 'CREATE TABLE vertices_tmp (id serial)';

FOR i IN EXECUTE 'SELECT srid FROM geometry_columns WHERE
f_table_name='''|| quote_ident(geom_table)||'''' LOOP
END LOOP;

srid := i.srid;

EXECUTE 'SELECT addGeometryColumn(''vertices_tmp'', ''the_geom'',
'||srid||', ''POINT'', 2)';

CREATE INDEX vertices_tmp_idx ON vertices_tmp USING
GIST (the_geom);

pre = '';
post = '';

I don't understand last assignations. In pgsql documentation, ther is
written that all assignations have to be written with ":=", not with
"=". What is the difference between "=" and ":=" ? I don't find any
information about "=".

Regards,

JKB