Here is an example index.theme file:

[Icon Theme]
Name=Birch
Name[sv]=Björk
Comment=Icon theme with a wooden look
Comment[sv]=Träinspirerat ikontema
Inherits=wood,default
Directories=48x48/apps,48x48/mimetypes,32x32/apps,scalable/apps,scalable/mimetypes

[scalable/apps]
Size=48
Type=Scalable
MinSize=1
MaxSize=256
Context=Applications

[scalable/mimetypes]
Size=48
Type=Scalable
MinSize=1
MaxSize=256
Context=MimeTypes

[32x32/apps]
Size=32
Type=Fixed
Context=Applications

[48x48/apps]
Size=48
Type=Fixed
Context=Applications

[48x48/mimetypes]
Size=48
Type=Fixed
Context=MimeTypes

The corresponding directory tree in the /usr/share/icons directory could look like this:

birch/index.theme
birch/scalable/apps/mozilla.svg
birch/scalable/mimetypes/mime_text_plain.svg
birch/scalable/mimetypes/mime_text_plain.icon
birch/48x48/apps/mozilla.png
birch/32x32/apps/mozilla.png
birch/48x48/mimetypes/mime_text_plain.png
birch/48x48/mimetypes/mime_text_plain.icon

Where birch/scalable/mimetypes/mime_text_plain.icon contains:

[Icon Data]
DisplayName=Mime text/plain
EmbeddedTextRectangle=100,100,900,900
AttachPoints=200,200|800,200|500,500|200,800|800,800

And birch/48x48/mimetypes/mime_text_plain.icon contains:

[Icon Data]
DisplayName=Mime text/plain
EmbeddedTextRectangle=8,8,40,40
AttachPoints=20,20|40,40|50,10|10,50

In this example a lookup of "mozilla" would get the prerendered 48x48 and 32x32 icons before the SVG icons due to the order of Directories.