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.
The Deck Pole Bird Feeder Swing Arm is clamp mount style that can swing in and out with a horizontal 25" reach. A variation for very wide railings is the Deck Bird Feeder Pole Wide Clamp for railings larger than 2” up to 5.5” thick.
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. The Deck Bird Feeder Bracket Arm has about a 36” horizontal reach off of the railing and is very strong capable pole. The screw in styles are typically a little more secure than clamp styles because they do screw into an upright post or railing.
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.