3.4. Multiple images and the "Greenbank Convention"
In the case of the binary table vector representation, all the images contained in a given column of the table may not necessarily have the same coordinate transformation values. For example, the pixel location of the reference point may be different for each image/row in the table, in which case a single 1CRPn keyword in the header is not sufficient to record the individual value required for each image. In such cases, the keyword must be replaced by a column with the same name (i.e. TTYPEm = '1CRPn') which can then be used to store the pixel location of the reference point appropriate for each row of the table. This convention for expanding a keyword into a table column (or conversely, collapsing a column of identical values into a single header keyword) is commonly known as part of the "Greenbank Convention" for FITS keywords and is illustrated in the example header shown in Paper II (Table 9).
There are several restrictions which may be too limiting for the parameters of certain types of coordinates and, in particular, for the distortion parameters to be introduced in Paper IV. The limitation to 8 characters limits the number of columns to 999, the number of axes to 9, and the number of parameters to as few as 10 (numbered 0 through 9, for column numbers exceeding 99). To avoid this difficulty, we introduce the concept of a parameter array as a single column of a table. All the parameters of a coordinate are given up to the maximum dimension of the column (given by keyword TFORMn) with no omitted parameters. Such parameter arrays are signaled by replacing the _m in the table column name with _X.
The Greenbank and parameter-array conventions are not needed with pixel lists since they are used to represent a single image.