In the past, the terms Apptio TBM Unified Model (ATUM) and TBM Taxonomy have been used interchangeably. In 2016, Apptio began to delineate the difference between these two concepts.  ATUM is a specification that defines a comprehensive, best-practice financial model for IT.  ATUM is comprised of four key components of which the TBM Taxonomy is one.  

Specifically, ATUM includes the following:

TBM Taxonomy: The organization of the standard IT business model into cost pools (financial view), IT resource towers (IT view) and services (business view).

Key Drivers and Allocations: The drivers and relationships that define the cost composition and allocation of costs within the IT business model.

Data: The datasets and definitions required to construct the IT business model.

Standardized TBM metrics and KPIs: The metrics to measure the performance of IT.