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The old pre-R13 formats (from r1.1 to r11) have no objects and no sections, just tables, which we store in the sections indexed by the enum Dwg_Section_Type_r11.
BLOCKS LAYER STYLE LTYPE VIEW
With r11 came the additional tables:
UCS VPORT APPID DIMSTYLE VX
Since r13 all those tables are stored as table control objects and tablerecord objects.
From pre-r13 DWG’s these tables are imported as old r11 sections and
as new CONTROL objects, so that all entities are accessible via the
single BLOCK_CONTROL.model_space -> BLOCK_HEADER.entities iterator,
all layers via the LAYER_CONTROL.entries -> LAYER objects, and so on.
All blocks are accessed via all other model_space BLOCK_HEADER’s,
get_first_owned_block (BLOCK_HEADER). Each CONTROL object
holds a list of all table records, i.e. entries. Each table record
entry has a name and other common table fields.