Sub-Table permit to relate one or more table in order to cerate a tree structure to see as “unique object” to fill at-once.
Relation between table and sub-table are 1 -> N type, so, for each record in main table can be present one or more record in child table and so on.
All action of Insert,Update,delete are performed on DB just when main table row is stored..
Use those object with care because all managed data are in memory till them are not stored on DB. SubTable with a lot of data can affect performance. SubTable property:

Name
Sub-Table name

Label
Label(language based) to show user when the subtable is refered

TableName
Name of table to set as subtable.

Relation
Relation between table and subtable column. Syntax:

<Col1>=<ColSubTable1> && ... && <ColN>=<ColSubTableM>

Where
<ColX> is name of table column
<ColSubTableY> is name of sub-table column.
Related column must be same type.

ColumnPositionName
In order to set a sort order (modifiable) those property must be set to a NUMERIC column name