There is only one file type that uses the .XSIG extension:
XAdES XML Signature File
An XSIG file is an XML Signature file used to verify the authenticity of a document. It is saved in the XAdES format, which includes various signature and timestamp properties. XSIG files are primarily used in Europe to digitally sign sensitive documents.
The XAdES format is an extension of the XMLDSig XML syntax developed and maintained by the World Wide Web Consortium (W3C). The format is used for XML signatures that authenticate messages and documents.
One of the main improvements provided by the XAdES format is that electronic signatures are valid for a longer period of time. XSIG files are primary utilized in business transactions, which include purchases and contracts.
NOTE: XAdES is short for XML Advanced Electronic Signature.
You can open XSIG files with AutoFirma, which is a multiplatform desktop application that can sign and validate documents saved in the XAdES format. The application is developed by the Ministry of Finance and Public Administrations of Spain.
You can also open XSIG files with eCoFirma, but it is no longer available for download since it was replaced by AutoFirma. It can run on Windows, macOS, and Linux.
Updated: July 3, 2020