The string breaks down into two distinct parts defined by the HTTP/1.1 specification :

: This identifies the language as Thai ( th ) and the region as ( TH ).

: Developers use these strings to match user OS settings with available translation files.

: Used by clients to request specific data formats or localized error messages from RESTful APIs. Summary Table: Component Meanings th Language Code ISO 639-1 code for Thai TH Region Code ISO 3166-1 alpha-2 code for Thailand 0.9 90% weight ; high priority but not the absolute first choice

: The server looks for a version in US English ( q=1.0 by default).

The server analyzes this list to determine which version of a page to serve:

: Often written as q=0.9 , this is a Relative Quality Factor . It ranks user preferences on a scale from 0.0 to 1.0. A value of 0.9 indicates it is a secondary preference if a primary language (defaulting to q=1.0 ) is unavailable. 2. Practical Application: How Servers Use It