Can I use the TAB command for SAP fields and inputfields I have created, other than in tables?
Also, can I deactivate the history for one GuiXT inputfield only?

Concerning the TAB command for your own fields: yes, this is possible. Please use the notation [field label] in the TAB command. For example:

InputFIeld (1,50) "abc1" (1,60) size=8 name=f1
InputFIeld (2,50) "abc2" (2,60) size=8 name=f2
InputFIeld (3,50) "abc3" (3,60) size=8 name=f3
InputFIeld (4,50) "abc4" (4,60) size=8 name=f4
tab [Account number] [abc1]
tab [abc1] [abc2]
tab [abc2] [abc3]
tab [abc3] [abc4]
tab [abc4] [Account number]

Deactivating the history update is possible as well, and with the same notation:

HistoryUpdate [abc1] Off