Open Advanced - Settings or Configuration - Editor - Word wrap / tab settings. Here you can define the Tab stop value for files with a specific extension which is by default 2 for all other files with Default selected in list box above.

A horizontal tab character is not used as a replacement for a fixed number of spaces in no text editor supporting programming and scripting languages. A horizontal tab is used to align text at specific columns depending on tab stop value in case of not using normal spaces for this task like when having UltraEdit configuration setting Use spaces in place of tabs enabled and pressing TAB key.

A tab stop value of 2 means text alignment with tabs is done on the columns 1, 3, 5, 7, 9, ...
A tab stop value definition with 16, 20, 8 means alignment with tabs is done on the columns 1, 17, 37, 45, 53, 61, ...

Multiple tab stop values as in second example are useful for example for assembler files (label, instruction, comment) or CSV files using tab character as separator. The screen shots display a tab-separated values file for which it would make sense to define multiple tab stop values depending on length of data in each data column.