A basis of plea is a written document from a defendant when they have pleaded guilty but they disagree with parts of the prosecution’s version of events.
A defendant will write a basis of plea when the difference between their version of events and the prosecution’s is significant enough that it would make a difference in the sentence they would receive.
The basis of plea will be sent to the prosecution, who will decide whether they accept the version of events put forward by the defendant. If they do not, there will need to be a Newton Hearing, where a judge will decide which version is true.