Hi SP

With Umbraco, every website implemented can have different options, properties and tabs.

So there isn't a standard way of doing Navigation, it will be down to whomever implemented your site as to what will happen if you update this particular Sitewide call to action field on your particular site.

When I take over a site that I haven't built there is always a series of unknown things to try and unpick 'how something works', what I'll often do is an audit of the site to try and be sure what everything does, for example there might be properties that are no longer used, or developers have their own starter kit for building 'most sites' but don't actually then implement things if they are not needed :-(

You should in the Settings section be able to view Document Types, and find the underlying definition for the 'Navigation' Document Type - if you click on that Info tab in your screenshot, it should tell yo the name of the Document Type if it's not clear.

Then inside the Document Type, find the 'Sitewide call to action link' label for this property, it will have an 'alias' inside a padlock, usually it would be something like

sitewideCallToActionLink

but it could be anything.

Then I would search all the 'views' and code (if you have an associated visual studio solution for the site) - for this 'alias'

And that would find if it's used anywhere, and if it is, you'd hopefully see where and be able to see what the logic is and what happens if it's filled in

eg something similar to this you are looking for...

 if (Model.Content.HasValue("sidewideCallToActionLink")){
var siteWideCallToAction = Model.Content.GetPropertyValue("siteWideCallToAction");
<a class="btn btn-primary" href="@siteWideCallToAction.Url">@siteWideCallToAction.Text</a>
}

regards

Marc