Some ATCOs clear an aircraft for a visual approach in the following manner:
"ABC 123 cleared visual approach, do not descend below altitude 2000 feet until established on final". (another way of saying "cleared visual approach, descend altitude 2000 feet")
This could be due to noise abatement, avoiding aircraft to turn in too tight and avoid overflying noise sensitive areas. Indicating that you do whatever you like, but descending below 2000' should be done on the centreline, with the nominal glide-path. Ie a different way of saying "aim for a 7 mile final" (in the 2000 feet case).
Dr.