It seems you have a two part question: 1) how to set ST 2 as default VSTi (plug in) synth? and 2) once you do that how do you save a particular set of SampleTank INSTRUMENTS (SampleTank patches if you prefer that terminology)?
If that is what you want then:
First, and this is "just in case" and please don’t use SampleTank 2's DXi - the DirectX plug in version (DXI of ST) causes some intermittent issues so to be safe use only the VSTi.
If this is FIRST time trying to load SampleTank 2 VSTi , you will have to tell BIAB where to find it (see step 3 in fist pic). That means YOU will HAVE to KNOW where YOU told SampleTank 2 to install the VSTi.
If you are using a 64 bit OS (and you must be if you are using SampleTank 3!) then it will probably be somewhere under the C:\ Program Files (X86)\ folder.
It might be in the \IK Multi Media sub-directory, it might be in the \VST or \VstPlugIns sub-directories. IN any case, you are looking for a file called "SampleTank2.x.dll" (you are not looking for an .exe file, you are not looking for any other .dll, only this one)
Now that you have set ST 2 as the DEFUALT synth, picture 2 shows you how to save and load a "group" ( .tgs) file (this saves the plugin's settings). This .tgs file will be saved in your BB folder (x:\...\BB\DX Settings\) , the X:\... is where ever you have BIAB installed. The only reason to make a .tgs file is the same reason you save any config file - you might want multiple configurations or if anything ever happened to that perfect collection of settings - LOL.
Finally, you really DO NOT NEED to create a .tgs file if all you want to do is have BIAB load ST 2 as the default at BIAB start up and have BIAB remember the instruments you had selected within ST 2: now that you have ST 2 set as default and you have it loaded up with instruments the way you want close BAIB and re-open, the LAST DEFAULT settings are loaded.