Certain features, such as parameterized cells, require cell properties in physical mode. Cell properties are also used to save the grid/snap values in the top-level cell, and can be added by the user to support other applications.
The GDSII format has no provision for storing properties of cell definitions. In electrical mode, Xic uses the format extension mentioned above. We can't use extensions in physical mode, since that would make the files non-portable, so we have to fake it.
In releases prior to 2.5.66, the cell properties were saved in a dummy label. This label was written on layer/datatype 0/0 at the origin, and was given the text string ``CELL PROPERTIES''. Physical cells with properties would have this label added in GDSII output. When reading in the GDSII file, the label would be stripped, and the properties from the label object would be applied to the containing cell.
However, when using direct conversions from the Format Conversion panel from the Convert Menu, the file would be converted as-is, so that if converting to Xic native cell files (for example), the converted cells would contain the ``CELL PROPERTIES'' labels and would not have the properties set.
Release 2.5.66 and later no longer create a ``CELL PROPERTIES'' label. Instead, a ``SNAPNODE'' record with a PLEX number 0xffffff is created, at the origin on layer 0 with nodetype 0. This is an obscure data type that is more likely to be invisible in the GDSII database. Unlike a label, it should not be visible in most other readers.
The current release reader will still process the ``CELL PROPERTIES'' label if found, for backwards compatibility. In addition, it will also process these labels, and the SNAPNODE records, when doing direct conversions, so that the properties are assigned correctly in this case.
Neither construct is/was added to the output file if the StripForExport button or variable is active.
COMPATIBILITY WARNING
Xic releases prior to 2.5.66 will not process cell properties in
GDSII files created with this release and subsequent. Physical mode
cell properties are used by Xic to implement parameterized cells,
to save the grid parameters used in the top-level cell, and can be
added by the user for third-party purposes. Loss of cell properties
will cause parameterized cells to lose the parameterization feature,
but still behave as normal cells. Loss of the grid parameters may
require the user to reset these manually. Files read with older
versions will generate ``unsupported record type PLEX'' warnings
in the log file if any of the new-style records are encountered.