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Linking within DocBook documents can be confusing, so here is a summary:

Intra-document Linking

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<xref>

use to get chapter/section number from the title of the target

link, or xreflabel if defined at the target, or refentrytitle if target

is a refentry; has no close tag

http://www.oasis-open.org/docbook/documentation/reference/html/xref.html

linkend=

controls the target of the link/xref, required

endterm=

for <xref>, allows the text of the link/xref to be taken from a

different link target title

<link>

use to supply text for the link, only uses linkend, requires </link>

http://www.oasis-open.org/docbook/documentation/reference/html/link.html

can be embedded inside of <command>, unlike <xref>

External Linking

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<ulink>

like <link>, but uses a URL (not a document target); requires

</ulink>; if no text is specified, the URL appears as the link

text

http://www.oasis-open.org/docbook/documentation/reference/html/ulink.html

url=

used by <ulink> to specify the URL, required

Guidelines

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- For an internal link, if you want to supply text, use <link>, else

<xref>.

- Specific nouns like GUC variables, SQL commands, and contrib modules

usually have xreflabels.

- For an external link, use <ulink>, with or without link text.

- xreflabels added to tags prevent the chapter/section for id's from being

referenced; only the xreflabel is accessible. Therefore, use xreflabels

only when linking is common, and chapter/section information is unneeded.