Each element to be sorted must have some inherent text on which it will be sorted. This text is the base sort phrase for the element. For elements that have titles, the base sort phrase usually is the content of the <title> element. For elements that do not have titles, the base sort phrase might be literal content in the DITA source, or it might be generated or constructed based on the semantics of the element involved; for example, it could be constructed from various attribute or metadata values. Processors that perform sorting SHOULD explicitly document how the base sort phrase is determined for a given element.
The <sort-as> element can be used to specify an effective sort phrase when the base sort phrase is not appropriate for sorting. For index terms, the <index-sort-as> element can be used to specify the effective sort phrase for an index entry.
The details of sorting and grouping are implementation specific. Processors might provide different mechanisms for defining or configuring collation and grouping details. Even where the <sort-as> element is specified, two processors might produce different sorted and grouped results because they might use different collation and grouping rules. For example, one processor might be configured to sort English terms before non-English terms, while another might be configured to sort them after. The grouping and sorting of content is subject to local editorial rules.
When a <sort-as> element is specified, processors that sort the containing element MUST construct the effective sort phrase by prepending the content of the <sort-as> element to the base sort phrase. This ensures that two items with the same <sort-as> element but different base sort phrases will sort in the appropriate order.
For example, if a processor uses the content of the <title> element as the base sort phrase, and the title of a topic is "24 Hour Support Hotline" and the value of the <sort-as> element is "twenty-four hour", then the effective sort phrase would be "twenty-four hour24 Hour Support Hotline".