Jim
It may be some inconsistency in naming the feature sets. I have checked the Software Advisor on the Cisco site for versions of software for the 7206VXR. I have checked 12.2, 12.2T, 12.3, 12.3T, 12.4, and 12.4T. I find some versions that have IP, some that have IP BASE, and some that have IP PLUS. But I can not find any version that has both IP PLUS and either IP or IP BASE.
In general Cisco creates different feature sets to provide different levels of functionality. The base image for example might provide support for routing protocols of RIP and OSPF. With higher feature sets you would get support for IS-IS or BGP.
If you can provide some details about your environment, particularly the version of code you are talking about then we might be able to get into more details about differences in feature sets.
HTH
Rick
HTH
Rick