Tizen 2.3.1 SDK will be no more supported soon. Before Tizen 2.4 SDK, each version of SDK released only for the same version of Tizen platform. 

However, Tizen 2.4 SDK contains all the features of Tizen 2.3.1 platform as well as Tizen 2.4 platform. Now, developers can develop Tizen applications for Tizen 2.3.1 and 2.4 with Tizen 2.4 SDK.