TADIR : Difference between structure and table

Hello Gurus,

I am trying to find the difference between table and structure as referenced in any table in SAP.

For example , the TABL object BOM_CLASS_LOCK is stored as table in TADIR but it is a structure type not table and so the program , that I am developing , is giving me a runtime because it cannot fetch data from a structure.
Is there any other table which I can see the difference between BOM_CLASS_LOCK and MAST(which is a table) for example?

Thank you in order! 😊