<field_name> is one of the following: name, shared, project (post project locator to "project" to associate a VCS root with a specific project).
A "VCS root" is the setting configured in TeamCity UI. "VCS root instance" is the internal TeamCity entity which is derived from the "VCS root" to perform actual VCS operation.
If the VCS root has no %-references to a parameters, a single VCS root corresponds to a single "VCS root instance".
f a VCS root has %-reference to a parameter and the reference resolves to different value when the VCS root is attached to different configurations or custom builds are run, a single "VCS root" can generate several "VCS root instances".