MatOb-Header
Matrix objects' headers can be set in the Matrix Dimension and Labels dialog (Select Matrix: Dimension/Labels from the menu). The Z Headers tab is a table-like 2D grid that allows you to enter matrix headers like Long Name, Units, Comments, etc. These headers, such as Long Name, can be shown in matrix thumbnails or used as Z-axis labels in 3D plots.
When editing Matrix object labels, you can right-click on the cell and bring up the following options: