key

leading character for terminal commands:
$ command to run in the terminal as a user
# command to run with sudo or as su (super user)

placeholder values:
$ command --argument [required placeholder value]
$ command --argument [[optional placeholder value]]
$ command --argument [required placeholder value [optional placeholder value]]

or vs xor:
or means a or b but also a and b
xor only means a or b

shortening ‘very’:
this is merely an idiosyncrasy of the author
outside of text formatted in this way v == very