As of build 5143 (December 19, 2022), eXtyles is compatible with 64-bit Office. If you are currently using an eXtyles build older than 5143 and you need an update that works with 64-bit Office, please contact eXtyles support.
Older builds of eXtyles, like many other third-party Microsoft Office add-ins, were written as a 32-bit application. 32-bit applications are not compatible with 64-bit versions of Office. You can find technical details about this limitation in these Microsoft articles: Compatibility Between the 32-bit and 64-bit Versions of Office 2010 and 64-bit Editions of Office.
For most Office users, and especially for those who primarily use Word and Outlook, 32-bit and 64-bit Office are functionally identical. However, the 64-bit architecture does provide additional power that can benefit power users of Excel and/or Access. IT departments may also prefer to install 64-bit Office because of its default security features.
Not sure what version of Word you're using? Learn more here.