Extra Information are normally all descriptive fields and 
Special Information are Key flexfield.
Extra Information , wch stores additional information 
whereas Special Info Identifies the entities.

1. EIT s are Descriptive Flexfields where as SITs are Key 
Flexfields

2. Special information is stored in a key Flexfield 
combination table where the combination of segment values 
must be unique whereas Extra information is stored in a 
descriptive Flexfield table where the combination of 
segment values does not need to be checked for  uniqueness

EIT and SIT Both are provided by Oracle to Capture Extra 
Information. Basic differences would be EIT is similar like 
an DFF and also defined using DFF Defination Screen.
SIT is KFF and defined using Personal Analysis KFF 
Defination screen. 
SIT generally used at Person Level, EIT can be defined at 
person,assignment,contact,element,location and Job Level.
There are 2 columns date_to and date_from in SIT while no 
such columns are there in EIT.