> //figure/preceding-sibling::*[1][figure]

The default axis is "child::", so this should be the same as

//figure/preceding-sibling::*[1][child::figure]


But you said you want the <figure> that has an immediate preceeding
sibling. The path

//figure/preceding-sibling::*[1][self::figure]

picks up the preceding sibling itself, not the <figure> that has the
preceding sibling. Perhaps you want something like

//figure[preceding-sibling::*[1][self::figure]]


> I wanted to isolate the figure element that has a figure as an
> immediate preceding sibling.
> 
>        <section role="All_models">
>             <title>All models</title>
>             <figure role="Harness_multi-plug">
>                 <title>ECU harness multi-plug</title>
>                 <mediaobject>
>                     <imageobject>
>                         <imagedata fileref="78650"/>
>                     </imageobject>
>                 </mediaobject>
>             </figure>
>             <figure role="Circuit_diagram">
>                 <title>System circuit diagram</title>
>                 <mediaobject>
>                     <imageobject>
>                         <imagedata fileref="31544"/>
>                     </imageobject>
>                 </mediaobject>
>             </figure>
>         </section>
> 
> I settled on //figure/preceding-sibling::*[1][figure] but it gave back
> nothing and would only work when I used the explicit form of the self
> axis.
> 
> I settled on //figure/preceding-sibling::*[1][self::figure]
> 
> However if I want the title element immediately preceding figure I
> don't need to use the explicit form. This works
> 
> //figure/preceding-sibling::*[1][title]
> 
> So what is going on in this particular case (because I don't
> understand the difference), and in general when is explicit self
> required.