Why end some selectors in `:alias` and others don't?

Question

Looking at the documentation of trinidad skinning selectors I noticed that some selectors end in :alias and others don't. For example:

.AFLabelDisabled:alias  Aliased style class that is included in the label selector for all the form components when they are disabled. It is a quick way to style disabled form components' labels the same, instead of for each component: .AFLabelDisabled:alias instead of e.g., "af|inputText:disabled af|inputText::label".
.AFErrorIconStyle   Style class that styles the .AFErrorIcon:alias icons.

There is even this selector, with and without the :alias:

.AFRequiredIconStyle    Style class that styles the .AFRequiredIcon icon.
.AFRequiredIconStyle:alias  Alias style class that styles the required icon for form components. It is included in the ::required-icon-style pseudo-element for the form components (e.g. af|inputText::required-icon-style)

I read the devguide on alias skinning, but that didn't help.

What exactly is the :alias for?

Answer 1

Directly from the docs

If you want to change the font-family to Arial for everything, then you need to do a replace of every use of Tahoma to Arial. Well, in skinning you don't need to do this. You can use the :alias feature. A skin selector that ends in :alias is one that never gets written out to the generated CSS-2 stylesheet. Instead it is included in other skin selectors and is useful in that you can change css properties in one place, not in tens or maybe hundreds of places.