When I read a JT file using JT Open Toolkit, then save the loaded data using JtkCADExporter::exportJt, and then read the resulting JT file using other CAD tools, these tools show that the root assembly has a property TOOLKIT_CUSTOMER which contains my license number (Sold_To_ID). Is it an actual property or something else? How can I access this data using the Toolkit? How can I disable the inclusion of this data inside the JT file on saving?
You may be referring to the infoSegment properties added to JT files with the 10.2 version. There are two methods described in the docs
addInfoSegmentProp - Add key-value pair info segment property to the JT file. Fails if there is an existing key. Prohibited from inserting a key which prefix is "JT_PROP". All Info Segment Properties are always authored with hidden visibility.
addInfoSegmentPropTranslator - Add two info segment properties to the JT file that describe the name and version of the translator used. JT_PROP_TRANSLATOR_NAME JT_PROP_TRANSLATOR_VERSION All Info Segment Properties are always authored with hidden visibility.