The Data Matrix is a data-collection mode in LabQuest (v. 1.5 or newer) and LabQuest 2. It was created to improve data collection in the field by addressing several key issues. These include:

1. Collecting data at multiple locations and over multiple days, all in one file.
2. Collecting data from more sensors than there are ports available.
3. Collecting data from certain combinations of water quality sensors that previously could not be used together due to electrical interference.

Detailed instructions can be found at:
http://www2.vernier.com/manuals/vernier-labquest-data-matrix-instructions.pdf

Related:
What do I need to do to prepare for outdoor data collection?
How do I map data collected on a LabQuest or LabQuest 2?