The .clr (pronounced "clear") or Continuous Layer Representation file format is an ASCII format to store velocity models that are continuously defined by radius or depth dependent polynomials and their parameters. Prominent examples for such models are PREM (Dziewonski & Anderson, 1981) and IASP92 (Kennett & Engdahl, 1991). The advantage of such model definition is that the model is uniquely defined at any depth and that there is no discussion about which kind of interpolation to use. A layering with first order discontinuities is still possible, since each polynomial has a well defined range of depth in which it is valid. The planet may consist of an arbitrary amount of such layers (what even allows a cubic spline representation of arbitrary velocity profiles).
The .clr format is a human readable ASCII format which is based on the Keyword Line concept used for some data in the .nd format. The .clr format is much more flexible than the .nd format and probably easier to read and write for humans. But as the .nd format, the .clr format is a line oriented format: Information is subdivided into blocks which are on separate lines.
Information is tagged by Keywords and Modifiers which define the type of information given in a line. This allows a very flexible file formatting.
The format definition is somewhat lengthy, but the resulting syntax and structure is quite simple and straight forward. I recommend to look into a .clr file before reading the format specification (see the Files You Need Section for examples).
It is important to understand that .clr files are not formatted by Format Strings as common in FORTRAN programming.