Not sure if I understand the question but 321682 is linked to 421682 in the breakends. You can kind of imagine the breakends having little "feet" which I sometimes call "directional feet" not sure if that helps

We have a pseudocode in javascript that helps parse breakend strings

for example parseBreakend('G]2:421681]') results in

{
    "MateDirection": "left",
    "Replacement": "G",
    "MatePosition": "2:421681",
    "Join": "right"
}

this says, the MateDirection is left because the square bracket points to the left and the "join" is right because it is after the letter G

another example parseBreakend('[2:421682[T') results in this

{
    "MateDirection": "right",
    "MatePosition": "2:421682",
    "Join": "left",
    "Replacement": "T"
}

The code for doing this looks like this (from our @gmod/vcf-js library)

  function parseBreakend(breakendString) {
    const tokens = breakendString.split(/[[\]]/)
    if (tokens.length > 1) {
      const parsed = {}
      parsed.MateDirection = breakendString.includes('[') ? 'right' : 'left'
      for (let i = 0; i < tokens.length; i += 1) {
        const tok = tokens[i]
        if (tok) {
          if (tok.includes(':')) {
            // this is the remote location
            parsed.MatePosition = tok
            parsed.Join = parsed.Replacement ? 'right' : 'left'
          } else {
            // this is the local alteration
            parsed.Replacement = tok
          }
        }
      }
      return parsed
    }
    // if there is not more than one token, there are no [ or ] characters,
    // so just return it unmodified
    return breakendString
  }