Symbolic links link by pathname rather than inode number. As you know, each pathname is a unique file on a system. Because of this, it is possible to create symbolic links across file system boundaries. Try to create symbolic links using following command:
$ touch /home/you/file1
# ln -s /home/you/file1 /tmp/file2
Find out inode of both file1 and file2
# ls -i /home/you/file1
# ls -i /tmp/file2
As you can see inode number are unique to each file. So it is possible to create symbolic links across file system boundaries. Please note that in above example both /tmp and /home are two different file systems.