What is read/write mode?

Irie Pascal Programmer's Reference Manual

The mode in which a file variable is opened specifies which operations can be performed on the file associated with the file variable, and also what happens to the file when the file variable is opened. If a file variable is opened in read/write mode then operations on the file associated with the file variable are allowed as follow:

Values can be retrieved from the file.
The values in the file can be changed.
New values can be added to the file.

Also if the file already exists when the file variable is opened then its contents are deleted. If the file does not already exist when the file variable is opened then it is created, and is initially empty.

Irie Pascal Programmer's Reference Manual