What is append 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 append mode then operations on the file associated with the file variable are allowed as follow:

Values can not be retrieved from the file.
The values in the file can not 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 preserved and the current file position is set to the end of the file. 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