Light Duty (LD) or Heavy Duty (HD)
If you have a single monitor, choose between the LD or HD version of the WorkFit-S. The LD version is usually the correct choice.
If you have an oversize monitor that is (> 24" or > than 18 lbs), an all-in-one computer or an iMac (which will also require an adapter) order the HD version.
Note that LD versus HD is not a choice between less durable and more durable. Both versions come with Ergotron's 5 year warranty. The difference is in the lift mechanism which is gauged for two different weight ranges. A WorkFit-S HD will not work properly with most standard monitors so normally you should select the LD.