In the description of the
/tikz/shape node option on p. 216 of the TikZ & PGF manual for version 3.0.1a states:
no default, initially
It goes on to explain that this option selects
the shape either of the current node or, when this option is not given inside a node but somewhere outside, the shape of all nodes in the current scope.
What is the difference between an option that has a default value and one that has initially a certain value? If instead of initially being
/tikz/shape would be
rectangle by default, what would change in its behavior and the way it can be used?
Can someone please explain and demonstrate the differences with a minimal example?
The present question is similar to this one. However that question deals with PGF (which I am unfamiliar with), whereas the current question deals with TikZ.
PGF and TikZ are two separate levels. Consider the following quote from the manual (p. 221):
Both PGF and TikZ support such multipart nodes. On the lower level, PGF provides a system for specifying that a shape consists of several parts. On the TikZ level, you specify the different node parts by using the following command:
The point of this quote is to show that the same feature can have different manifestations in PGF vs. in TikZ and it is reasonable to ask how a certain feature manifests in one level and how it manifests in another level, and this would constitute two different questions, just as a question about TikZ's
\nodepart command can coexist with a question about PGF's system for specifying that a shape consists of several parts without any of these questions being a duplicate of the other.
My question is: Is it possible to explain the difference and to give a minimal example using only TikZ concepts and syntax without resorting to low level PGF? Is the difference perceptible from a high-level user's perspective who only knows TikZ? (I am such a user.)