Study tagging file (STF) is an xml which contains the group of files for a particular study.
Study tagging files are required and applicable in US. Study tagging files applies to only module 4 and module 5 of eCTD.

Why STF ?

The eCTD backbone files do not contain enough information on the subject matter of several documents (e.g., study report documents) to support certain regulatory uses. This additional information is provided in the STF. An STF should be provided with the submission of any file, or group of files belonging to a study in Modules 4 and 5.

So each file in module 4 and module 5, you should place the STF for the specific study in the module folder with the corresponding study files for US submission.

In which studies is STF required ?

STF has a xml file, and in it the different documents of study reports are attached and linked. And it is required for all study reports in module 4 and module 5. However, According to FDA, even if we have individual/seperate single study files, still the STF should be used in and eCTD submission, except for clinical tabular listing and literature reference (i.e. only this two documents do not require STF).You can refer to the STF specification file version 2-6-1, which is easily available from web.