Template:Dau
For instance, if you have a template which contains a line like [[{{{story title|}}}|{{dau|{{{story title}}}]], you cannot use {{da}}.
To understand why, let's look at a concrete example. If we were to set story title equal to An Unearthly Child (TV story), then {{da}} would unpack in this way: [[{{{An Unearthly Child (TV story)|}}}|[[An Unearthly Child]]]]
See what's going on after the pipe? It's putting a link within the link. If, on the other hand we use {{dau}} — again, like [[{{{story title|}}}|{{dau|{{{story title}}}]] — then we get [[{{{An Unearthly Child (TV story)|}}}|An Unearthly Child]], An Unearthly Child is returned, unlinked — exactly what we want in this circumstance.
If you want more or less the same thing, but in a way that works better in SMW tables, you should consult {{da smw}}.