The + key on the number pad will move from one field to the next but it is technically what is called a "Field Exit" key:  It deletes field contents from the cursor position to the end of the field, then moves on to the next field.  If your cursor is already at the end of a field when you use it, it may appear to behave like the Tab key, but just be careful so you don't accidentally delete something!
 
In general, use Tab when just moving from one field to the next, and save the + key on the number pad for when you are actually keying fresh data into empty fields.  (It’s also handy for quickly clearing all of the content from many fields on a screen.)

NOTE: Prior to the ING upgrade in 2014 the + key did work exactly like Tab, but that functionality is no longer supported by the core LegaSuite client software.