After you create a local or global variable, you can read data from a variable or write data to it. By default, a new variable receives data. This kind of variable works as an indicator and is a write local or global. You also can configure a variable to behave as a data source, or a read local or global.
On the block diagram, you can distinguish read locals or globals from write locals or globals the same way you distinguish controls from indicators. A read local or global has a thick border similar to a control. A write local or global has a thin border similar to an indicator.