The cells in the right column of the table shown in figure 4.5 have different leading or padding. Observe that the default leading of the content of a cell in text mode is equal to the font size. Internally, the leading is set like this: This is different from the default leading of a Phrase object outside a PdfPCell.
There can be a lot of differences between a UDF and stored procedure, as the prior link has shown. Another difference is the UDF can be directly selected from, whereas a stored procedure requires one to insert the results into a tempory table or table variable.
It's internal in the sense that data is stored in the primary memory for quick access by the processor. Secondary storage stores programs and information that isn't needed immediately. Another difference is that primary is generally faster than secondary memory. This is mostly to cut down on the cost of the computer. Fast memory is expensive and seeing that fast memory is more necessary for primary storage it is implemented there.