In general, ILMT is required for sub-capacity licensing. ILMT is a free product whereas BigFix is paid and performs more functions than is necessary for IBM sub-capacity licensing.
What may be causing some confusion is around the implementation of ILMT. In order to install ILMT, a customer must install the License Metric Tool server and its database, the BigFix server and its database, a BigFix console, and a BigFix client on each server that the customer wants to collect software inventory data. These BigFix components are free of charge and should be included with the ILMT installation packages--thus a *full* implementation of BigFix is *not* required.
One last comment to make is that ILMT is designed to only survey and measure PVU and RVU (core-based) IBM products. It will not gather or measure information about non-PVU products. This is important to note if the customer has a misconception that ILMT will inventory all of their IBM products.
Additional Research:
https://www.ibm.com/support/knowledgecenter/en/SS8JFY_9.2.0/com.ibm.lmt.doc/Inventory/planinconf/t_p...
https://www.ibm.com/support/pages/faq-license-metric-tool-and-bigfix-inventory
https://www.ibm.com/developerworks/community/wikis/home?lang=en#!/wiki/IBM%20License%20Metric%20Tool...
Hope this helps.
