A scantable has header information and data (a scantable is actually an AIPS++ Table and it is stored in Memory when you are manipulating it with ASAP. You can store it to disk and then browse it with the AIPS++ Table browser if you know how to do that !).
The data are stored in columns (the length of a column is the number of rows/integrations of course).
Two important columns are those that describe the frequency setup. We mention them explicitly here because you need to be able to understand the presentation of the frequency information and possibly how to manipulate it.
These columns are called FreqID and RestFreqID. They contain indices, for each IF, pointing into tables with all of the frequency information for that integration. More on these below when we discuss the summary function in the next subsection.
There are of course many other columns which contain the actual spectra, the flags, the Tsys, the source names and so on, but those are a little more transparently handled.