you should have put your subject line into Google, the same question was asked and answered in 2014 and was the very first result:

what is the difference of stock type X and 2

Dear Hasne Matanog,

Jürgen is right, it worth to search before posting a question, you might get the answer even more faster than waiting for replies of Community members. (Additional info: the mentioned search engine is looking through SAP Knowledge Base Articles as well.)

I have checked the Data element MIGO_INSMK (Short Description Stock Type) and here is the complete F1 Help document:

"Stock Type
Indicates the stock type to which the quantity is posted.

Use
Possible stock types are:

• unrestricted-use_stock (blank, F)
• stock in quality inspection (X, 2)
• blocked stock (S, 3)

Choose the relevant indicator for the required stock type.

Dependencies
Due to earlier release levels, there are only two valid values (each with the same effect) for each stock type."

Have a nice day,

Albert