Substantial portion is a legal term. Its exact definition will depend on jurisdiction, be subject to interpretation and possibly including subjective analyses. When in doubt, ask a lawyer. To err on the safe side, assume that it applies to any portion, no matter the size.
To give you a random example, here's what Wikipedia has to say for Canadian copyright law (neither I, nor Wikipedia, are lawyers and cannot give legal advice, obviously):
What is a Substantial Part?
The Copyright Act does not require that an entire work be copied in order to find that a copyright holder’s rights have been infringed. Copying a substantial part of the work will be sufficiently actionable. In determining whether a substantial part of a work has been copied, the court must exclude from consideration any part of the work not properly the subject matter of copyright under s.3 of the Act. The analysis is largely fact-driven, and the courts will consider both qualitative and quantitative matters. Some of the matters that have been considered include:
a) how central was the element taken to the first work?
b) do the selected parts constitute an essential characteristic of the work?
c) would people who see the copy recognize the source?
d) was the part taken used in a manner that would create a substitute to the first work and thereby jeopardize its economic exploitation? [economic damages can be considered]