The LicenceKeyControl is a simple composite control that provides a handy fragment of UI for capturing a licence key and displaying associated error messages in a module settings control. It has no functionality beyond that, as it calls back to the settings control in a LicenceStatusRequired event to perform the actual licence key validation.
Regarding DNN6 design patterns / styles, the LicenceKeyControl is really skin-agnostic: it has a couple of CssClass properties that you can set in Settings.ascx and then use in your skin css. CssClass is applied to the fieldset tag that wraps the whole control. ValidatorCssClass is applied to a validator control that LicenceKeyControl uses to display error messages. Beyond that it will pick up the default styling for fieldsets, textbox, labels and span elements from your skin.
Similarly, the validation messages are set as properties on the control, so your settings control can do the work of localising them as required. The sample Licence Master Client PA project, illustrates one way of doing this:
private void SetupForm()
LicensingUtility lu = new LicensingUtility(Assembly.GetExecutingAssembly(), LicenceKey, Request.Url.Host);
lnkDownloadUrl.NavigateUrl = lu.GetApplicationDownloadUrl();
lnkDownloadUrl.Text = lnkDownloadUrl.NavigateUrl;
lkcLicenceKey.InvalidLicenceMessage = Localization.GetString("InvalidLicence.Message", LocalResourceFile);
lkcLicenceKey.RegistrationRequiredMessage = string.Format(Localization.GetString("RegistrationPending.Message", LocalResourceFile), lu.GetApplicationRegistrationUrl());
lkcLicenceKey.VerificationRequiredMessage = string.Format(Localization.GetString("VerificationPending.Message", LocalResourceFile), lu.GetApplicationRegistrationUrl());
Hope that helps.