Similarly to variables, globals can be created using master globals. Globals only tracks a single value, each time the value is updated, the value is overwritten, and only keeps the last value.  Once a master global is defined, it is automatically added to the definition just as a master variable. 

Master globals UI for defining globals look as follows:

Add globals by entering the name of the variable and by selecting a data type for the variable. Remove any globals by selecting them from the list. The 'Variables / Globals have default values based on type' functionality in the Process Instances page.

• Variables and globals have '0' for the 'Number' data type
• Variables and globals have ' ' for the 'String' data type
• Variables and globals have 'Current date and time' for the 'Date' data type
• Variables and globals  have 'false' for the 'Boolean' data type