Multis 0 -15 store the Preset information for each Part with the Multi. Multis 16 - 127 store references to the Presets in the RAM and ROM banks. This is explained in the Virus TI Quickstart manual:
Quote from Virus TI Quickstart manual, p.19
The Virus TI has 128 slots for storing your Multi set-ups. The first 16 (0-15) of these are ‘embedded’ Multis, by which we mean that they store all the data for the patches they use, so any edits you make to the sounds are automatically preserved when you store the Multi, so you need never worry about breaking a preset. Multis numbered 16 and upwards do not store the individual patch data, but instead refer to patches stored in the Single RAM/ROM banks.
The Virus TI is a complex device. Reading the manuals and working through the tutorials is really necessary to understand the basic concepts and learning the advanced features in order to unlock the full potential of this wonderful instrument.