Choose the screw length around the actual timber joint. The 50mm version is the shortest option, 75mm suits joints needing more reach without using 100mm, and 100mm is intended for more substantial timber sections.
The three Impact Professional Wood Screw sizes use the same 5.0mm diameter, but their different lengths are intended for different timber thicknesses.
The 5.0 x 50mm screw is the shortest option and is intended for joints where the 75mm or 100mm versions would provide unnecessary extra length. It has a 30mm threaded section.
The 5.0 x 75mm screw is the intermediate option. The supplied guidance specifically positions it where a 50mm fixing does not provide enough reach but a 100mm screw would be excessive. Its threaded section is 40mm.
The 5.0 x 100mm screw is the longest of these three and is intended where the fixing needs to pass through a more substantial timber section before engaging with the timber behind it.
All three are hardened steel, zinc and yellow passivated fixings with a Pozi recess, ribbed double countersunk head, milling shank and Type 17 point. Choose the length from the actual pieces being joined rather than using one screw size for every fencing job.