(Heavily edited one hour after original posting)

2-wire typically refers to a field device that is a 2-wire, loop powered passive device that needs +24Vdc on its (+) terminal in order have the electrical power to drive the electronics and produce a 4-20mA signal.

4-wire typically refers to a field device where 2 wires provide external power to the device (could be batteries internally) and where the device provides an active, powered 4-20mA output signal on 2 other wires.   If you can read the current from a device's 4-20mA output  with an ordinary DVM set to DC current, it is an active signal, from a 4-wire device.
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Many handheld mA generators can be either 2-wire or 4-wire, but I suspect you used the 4-wire mode, with the generator batteries supplying power to the signal.

If that's the case, then you have to wire your signal as a 4-wire signal, which is very confusing, as shown on page 12 of the manual, because the 4-wire diagram labels the DC power terminals as (+) and (-), but does not label current signal as +/-.   
https://cache.industry.siemens.com/dl/files/183/109475183/att_837325/v1/et200sp_ai_8xi_2_4_wire_ba_manual_en-US_en-US.pdfI think
I+ is the current signal input
6A/8A are AUX terminals but appear to connect to M, ground
Uv should be a voltage supply, by the convention U for voltage which is used to power the 4-wire field device.

I would wire a 4-wire device, like a battery powered generator or a 4-20mA active signal as:
signal (+) to I+
signal (-) to 6A (Aux) or M ground

The wiring seems to be confirmed by observations by contributor ingenieur_fabrication_dbm on this thread
https://support.industry.siemens.com/forum/ww/en/posts/4-wire-connection-in-6es71346gf000aa1/239638/?page=0&pageSize=10#post1222536

Although his example with with a different model p-n AI card, it is also 2-wire/4wire AI card, where
Uv is the DC power source
I+ is the current signal input