is it a GAAP principle?
what are the main parameters of the guideline?
how have companies complied with it?

SOP 98-1 is a Statement of Position issued by the American Institute
of Certified Public Accoutants. It provides guidance on the
capitalization of software created for INTERNAL use. Statement of
Financial Accounting Standards No. 86 provides guidance on accouting
for software developed FOR SALE.

The main parameters of the guidance is that planning and inital
selections of alternatives are expensed as incurred.  After the
planning has been completed and coding has begun, these costs may be
capitalized (including internal labor--only those working directly on
the project--and keeping time sheets is important). External
consultants may also be capitalized. Capitalization stops when the
project is ready for its intended use. Training end-users are expensed
as incurred.

I recommend getting your hands on the SOP to develop an understanding.
It provides some good examples.

Regards, jdebitjr