During data processing in the transformation there are different structures and internal tables where the records are temporary stored.
The naming convention is really helpful.
YT_SC_1 -> T stands for Table and SC stands for source
YS_TG_1 -> S stands for Structure and TG stands for target
The following picture shows the sequence of the structures and tables for the records during the transformation process.
There are two other important structures. One is used in the start routine (SOURCE_PACKAGE) and another one in the endroutine (RESULT_PACKAGE).