When you press the Tab key the cursor will normally move five spaces to the right. All normal Tab sets are left justified i.e. when you type in columns the text will be left justified in the columns.

Before customising (creating to your own needs) tab settings click on Clear All to clear all the default tab settings.

Type the position of the tab setting (in cm) in the space under Tab stop position and then click on the Set button.
Then click on the circle of your choice under the Alignment section. Choose Decimal if you want a decimal Tab.

You sometimes need to type numbers and decimals in columns (e.g. Accounting exam papers and notes). In this case it is advisable to set your own decimal tab settings.