Nested structure

Can any one of you clarify this:

TYPES: BEGIN OF STRUCT1.

COLI TYPE I,

BEGIN OF STRUCT OF STRUCT2,

COL1 TYPE I,

COL2 TYPE I,

END OF STRUCT 2,

END OF STRUCT1.

TYPES MYTYPE TYPE STRUCT1-STRUCT2-COL2.

Please clarify why the last TYPES statement is used and how it works?