The MSDN Page at https://msdn.microsoft.com/en-us/library/textio.new.aspx just states "Specify one the following: " but the following list is missing.
I think the class did not change the possible mode values from 2009 to 2012 (those mode values are somewhat general across languages/frameworks anyway) so you may want to have a look at the page for AX 2009
The mode in which the file should be opened. Specify one the following:
R - Read
W - Write
A - Append (implies "W")
T - Translate (text)
B - Binary