When answering ArcGIS Desktop and ArcGIS Pro questions I know that I will often copy/paste a short paragraph or two from their Online Help.
For example, at https://gis.stackexchange.com/a/209427/115 I answered:
To do this try using Union which:
Computes a geometric union of the input features. All features and their attributes will be written to the output feature class.
followed by Summary Statistics which:
Calculates summary statistics for field(s) in a table.
You would use the identifier field from one of the original layers as the case item to summarize information from the other.
However, I sometimes see answers that provide attribution to the ArcGIS Online Help but copy much larger parts of its pages (perhaps 80%) and some of the pictures.
Does GIS SE provide any guidance on how much of a help page or blog posting may be copied before that action starts to cross-over into plagiarism and/or copyright infringement?