GDSII is by definition a hierarchical database. AutoCAD also support hierachy in the form of blocks and block insertions. GDSII requires at least one structure to contain data. By default we create a structure called TOP. So even if your AutoCAD drawing has no blocks the resulting GDSII will have a single structure named TOP.
If the AutoCAD file uses blocks then the same hierarchy is created in GDSII. For example, consider an AutoCAD file that has data at the "top" level along with 3 block definitions (B1, B2 and B3) and each block is inserted. Then the GDSII database will look like this:
Notes:
1. Block insertions may be scaled, rotated or mirrored. However insertions with different X and Y scales are automatically exploded since GDSII does not support such a concept.
2. GDSII structure names are case sensitive.
3. GDSII has restricted length and character rules. Block names should not exceed 32 characters and the only special characters that should be used are: $, _ and /. ASM 3500 will replace "illegal" characters with underscores.
4. The block insertion "layer" in AutoCAD affects how entities (within the block) belonging to layer 0 appear -- this has no equivalent in GDSII.