If an X-ray machine has a detector:
Apparently some dicom tags are not always available eg. (0018, 700a) Detector ID. At what point in time it is decided which dicom tags are stored in the dicom files. At the installation time of the maschine or is it subsequently expandable?
EDIT Is there any settings to switch on/off to push dicom tags into the file?
DICOM standard gives each tag a type, which indicates, whether it's mandatory or not. The tag You mention (0018, 700a) Detector ID is type 3 or optional. That means, that including or not including it is up to the vendor of the machine or software.
Whether it's configurable or not also depends on the implementation. The best way to find out, would be to look at the DICOM conformance statement of the equipment in question. They might list all the tags they include in their output with explanations where the values come from or whether they are configurable.