Think of the "?msource" code as a custom identifier.
You create this custom identifier as a way to identify a website/social network may it be Facebook, Google+, Twitter, the organizations website,  etc...
The identifier is created using Source Codes in Sphere.  Create the correlating source codes in Sphere. 

Using the steps listed in the link above lets say you create Source Codes for Facebook, Twitter and your organization's website.

You will then copy the source codes you created and append each one to the desired event page link in Sphere
Append the link with the following " ?msource=XXXX" where XXXX = the source code you created in Sphere. 
For example, below is a mock general donation page link:
Using the source codes you have created in Sphere, use the ?msource code to create custom donation link that would be pasted on each of the respective sites.

This example link for the donation page would be posted on Facebook

This one on Twitter

and this one on the organizations website.

Then when the link is clicked and a donation is made,  Sphere uses the msource code to identify where to donation came from.

*NOTE* Because the link already contains a "?",  I needed to change the "?" that precedes "?msource=..." to a "&" so that it reads "&msource=..." at the end of each link. If you do not do this the link will break and send you to a blank page.

*NOTE* The &msource code should only be appended to page URLs generated by Sphere: Single step donation pages, Registration pages, Thon events, CMS sites, etc...

You can then run a query in Sphere to see what donations where made using that source code/custom identifier:

1. Go to Control > Query
2. Click New > Select Payments Query
3. Expand Gift > Gift Related Information
4. Click Source Code > Click Add
5. Select your criteria (i.e Source Code Is one of ... etc)
6. Click Ok
7. Click Next
8. Select fields for output (i.e First name, Last Name, Total Gift Amount, Source Code, etc)
9. Click Finish and wait for the report to complete

*NOTE* The source code will not show up on the reporting until an activity record generates on the supporters contact record in Sphere.

How to run a report and filter by Source Code: