The user data values are encoded in shell variables named
prefix "NM_USER_". The variable name is an encoded form of the
data key, consisting only of upper-case letters, digits, and underscore.

The alternative would be something like

  NM_USER_1_KEY=my.keys.1
  NM_USER_1_VAL='some value'
  NM_USER_2_KEY=my.other.KEY.42
  NM_USER_2_VAL='other value'

contary to

  NM_USER_MY__KEYS__1='some value'
  NM_USER_MY__OTHER___K_E_Y__42='other value'

The advantage of the former, numbered scheme is that it may be easier to
find the key of a user-data entry. With the current implementation, the
shell script would have to decode the key, like the ifcfg-rh plugin
does.

However, user data keys are opaque identifers for values. Usually, you
are not concerned with a certain name of the key, you already know it.
Hence, you don't need to write a shell script to decode the key name,
instead, you can use it directly:

  if [ -z ${NM_USER_MY__OTHER___K_E_Y__42+x} ]; then
      do_something_with_key "$NM_USER_MY__OTHER___K_E_Y__42"
  fi

Otherwise, you'd first have to search write a shell script to search
for the interesting key -- in this example "$NM_USER_2_KEY", before being
able to access the value "$NM_USER_2_VAL".