[pomotom]

The difference between DS and TO can be
summarized as follows:
A DS deines which ields can be linked with the HTML template.
A TO determines which HTML elements are linked to which ields

from Mastering TypoScript

Data Structures (DS) and Template Objects (TO)
After the mapping of TemplaVoila, the configuration of the HTML template
is saved in two XML data structures (DS and TO) and apart from the infor-
mation on the areas selected, also contains information on what type of
objects or data these areas can take in.
Data structures (DS) and template objects (TO) are separate definitions,
but make use of each other. Data structures contain an abstract defini-
tion of areas, fields, and field types, comparable to the definition of the
fields in a database—just with a few additional "gimmicks".
Template objects refer to a DS and define the output of an element of the
DS. A TO, for example, contains information concerning which HTML files
are to be used for rendering and which fields from the data structure are
to be used at which points in the HTML structure. Multiple TOs can be
defined for a DS; the basis for the different output variations.
There are still more areas where DS and TO can be used. A DS is used
both for the definition of base layouts and of content types.

from "oficial" TYPO3 book

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