The ASC file format is defined by the type of data lines that appear in it, the format of these lines, and the order in which these lines occur. Data lines consist of several types:

• Blank or comment lines, which are ignored. The first non-blank character on a comment line is one of "#", "/" or ";".
• File preamble or file-description lines. These begin with "**". Usually these lines are ignored when processing the ASC file.
• Sample data lines. Each line begins with a number, representing the time of the sample.
• Event and data-description lines. Each line begins with a keyword, identifying the type of data in the rest of the line.

For sample-only ASC files, file structure is very simple. Only sample data lines are present. There is no data on what type of eye-position data or which eye produced the data. Recording blocks are separated by samples lines consisting of missing-value data (dots or the string specified with the "-miss" option). Gaps in the sequence of sample timestamps may also be used to determine sample block divisions.

For ASC files containing events (and optionally samples), the order of lines is carefully structured. The order of items in an ASC file is similar to that of a sorted EDF file. The file begins with a copy of the EDF file's preamble, with each line preceded by "**". The preamble reports the file version, date created, and any description from the application. Usually the preamble is ignored during analysis.

The sequence of events and samples in the ASC file follows strict rules. These are:

• START events mark the beginning of each recording block, and END events mark the end of each block. The START events also specifies which eye(s) data is present, and if samples, events, or both are present.
• Data-specification lines follow each START event. These specify the type of data in samples and events in the block, and allow flexible data processing without prescanning the file.
• All eye-movement samples and events occur between the START event and the matching END event.
• All events and samples appear in temporal order. That is, the timestamps of samples, end-time timestamps of eye-movement end events, and start-time timestamps of all other events will be the same or greater than any preceding data.
• Eye-data samples are nested between eye-movement start and end event. For example, the first sample in a fixation will follow the SFIX event for that fixation, and the EFIX event for a fixation will follow the last sample in the fixation. This allows on-the-fly classification of samples as the data file is read.