I am not aware that Microsoft has published a list of TFS icon symbols and their meanings.
It would be helpful to have such a list.
For example, what do the icons mean in the following image?
My guesses for these two symbols are...
1) green merge symbol means that a merged folder has yet to loaded into the workspace
2) green plus symbol means that a folder created in the workspace has yet to be checked into the server
For now there is not any official document describes the accurately meaning of the latest version control icon symbols.
This link explained some version control icons, however it's out of date. Some of the icons changed, some of the icons(such as you provided) are not in the table.
It's a good suggestion, I have created a uservoice for you. You can vote up for it to get more attention.
User Voice Of Team Services
Provide a list explaining the meanings of all TFS version control
icon symbols(Better Continuous Updates)
And for the meanings of the icons you provided, we can only inferred from the behavior of TFS server. Your guess is correct, a little supplement: green plus symbol also means the changes aleady in pending changes list.