1) Railing Clamp Mount styles are typically used on a standard 2x4” railing. That provides a good area for a clamp to secure itself to the railing. A variation for very wide railings is the Deck Bird Feeder Pole Wide Clamp for railings larger than 2” up to 5.5” thick.
The Extended Deck Pole Swing Arm and the Deck Bird Feeder Bracket Arm are basically the same with the exception of their mounting. They both have about a 36” reach off of the railing and are both very strong and capable poles.
2) Screw Mount style needs about a 4” flat surface to screw into to secure the pole. This can be a post or wide railing. This can be used when an unusual shaped railing or design does not allow a clamp style. A screw mount is good to secure the pole where you have 4" flat surface to screw into and don't mind doing so. Our Deck Bird Feeder Bracket Arm has a long 36" reach and can typically be screwed into the upright posts that you have. The Deck Bird Feeder Bracket Arm is more secure because it screws into the railing or post.
Metal railings are often more complex than say a standard 2x4 shape. When a metal railing with smaller ½” square balusters are present, the Deck Pole Iron Railing Mount works great. These Baluster mounts with shepherd hook poles are very strong, more secure than a clamp or screw mount and can support the most weight.
Deck Pole Baluster Mount Deck Pole Iron Railing Mount
In Summary: A lot of the Deck Bird Feeder Poles are made to clamp onto a 2” thick 2x4 railing. Depending on the railing and deck you have, it might be better to screw into a post or attach onto a baluster instead.