Evaluation mode licenses are available by default for every asset. While in evaluation mode, you can explore all the features of the product that is associated with the asset.
When you install an asset, its default license is evaluation mode. Evaluation mode licenses expire after 60 days. If you assign a license key to the asset before its evaluation period expires, the time available in the evaluation period is decreased by the time already used. To explore the entire set of features that are available for an asset, you can set it back to evaluation mode, and use it for the remaining evaluation period.
For example, suppose that you use an ESXi host in evaluation mode for 20 days and then assign a vSphere Standard license key to the host. If you set the host back in evaluation mode, you can explore the entire set of features that are available for the host for the remaining evaluation period of 40 days.
You can track the number of remaining days available for the evaluation period of an asset from the vSphere Web Client.