The Rockwell Automation Allen-Bradley (AB) sensor switches mentioned are part of the 440N, which are proximity sensors commonly used in industrial automation applications. Here's a breakdown of each sensor switch you mentioned:

1. Rockwell 440N-Z21S16H AB Sensor Switch:

   This particular sensor switch is not valid or recognized. It seems to be a typographical error or an incorrect part number.

2. Rockwell 440N-Z21S17B AB Safety Switch:

   The 440N-Z21S17B is a model of Allen-Bradley safety switch from Rockwell Automation. Safety switches are used to monitor and ensure the safety of machine guards, doors, or access points in industrial settings. This particular model may have specific features suited for safety applications.

3. Rockwell 440N-Z21S26A AB Proximity Switch:

   The 440N-Z21S26A is a model of Allen-Bradley proximity switch from Rockwell Automation. Proximity switches are used to detect the presence or absence of objects without physical contact. This model may have specific specifications and features, such as sensing range and output type, suitable for various automation and sensing applications.