Understanding start, 2>nul, cmd, and other symbols in a batch file

Question

Following up from this topic on here, I'm trying to understand what's happening "behind the scenes" from a suggested answer. I don't understand what 2>nul or 1>nul is supposed to do. And I tried to decipher what the symbols in the start /b line is doing, but I am really clueless here. I need a step by step approach on that one if you don't mind.

What's happening in this part of the code?

    2>nul del %lock%!nextProc!
    %= Redirect the lock handle to the lock file. The CMD process will     =%
    %= maintain an exclusive lock on the lock file until the process ends. =%
    start /b "" cmd /c %lockHandle%^>"%lock%!nextProc!" 2^>^&1 !cpu%%N! !cmd!
  )
  set "launch="

And this:

    ) 9>>"%lock%%%N"
  ) 2>nul
  if %endCount% lss %startCount% (
    1>nul 2>nul ping /n 2 ::1
    goto :wait
  )

2>nul del %lock%*

Copy of suggested code in full:

@echo off
setlocal enableDelayedExpansion

:: Display the output of each process if the /O option is used
:: else ignore the output of each process
if /i "%~1" equ "/O" (
  set "lockHandle=1"
  set "showOutput=1"
) else (
  set "lockHandle=1^>nul 9"
  set "showOutput="
)

:: Define the maximum number of parallel processes to run.
:: Each process number can optionally be assigned to a particular server
:: and/or cpu via psexec specs (untested).
set "maxProc=8"

:: Optional - Define CPU targets in terms of PSEXEC specs
::           (everything but the command)
::
:: If a cpu is not defined for a proc, then it will be run on the local machine.
:: I haven't tested this feature, but it seems like it should work.
::
:: set cpu1=psexec \\server1 ...
:: set cpu2=psexec \\server1 ...
:: set cpu3=psexec \\server2 ...
:: etc.

:: For this demo force all cpu specs to undefined (local machine)
for /l %%N in (1 1 %maxProc%) do set "cpu%%N="

:: Get a unique base lock name for this particular instantiation.
:: Incorporate a timestamp from WMIC if possible, but don't fail if
:: WMIC not available. Also incorporate a random number.
  set "lock="
  for /f "skip=1 delims=-+ " %%T in ('2^>nul wmic os get localdatetime') do (
    set "lock=%%T"
    goto :break
  )
  :break
  set "lock=%temp%\lock%lock%_%random%_"

:: Initialize the counters
  set /a "startCount=0, endCount=0"

:: Clear any existing end flags
  for /l %%N in (1 1 %maxProc%) do set "endProc%%N="

:: Launch the commands in a loop
  set launch=1
  echo mem=1m 2m 3m 4m 6m 8m 12m 16m 24m 32m 48m 64m 96m 128m 192m 256m 384m 512m 768m 1024m
  echo o=2 3 4 5 6 7 8 10 12 14 16 20 24 28 32
  echo s=off 1m 2m 4m 8m 16m 32m 64m 128m 256m 512m 1g 2g 4g 8g 16g 32g 64g on
  echo x=1 3 5 7 9
  for %%x IN (9) DO for %%d IN (1024m 768m 512m 384m 256m 192m 128m 96m 64m 48m 32m 24m 16m 12m 8m 6m 4m 3m 2m 1m) DO (
    set "cmd=7z.exe a teste.resultado\%%xx.ppmd.%%dd.%%ww.%%ss.7z .\teste.original\* -mx=%%x -m0=PPMd:mem=%%d:o=%%w -ms=%%s"
    if !startCount! lss %maxProc% (
      set /a "startCount+=1, nextProc=startCount"
    ) else (
      call :wait
    )
    set cmd!nextProc!=!cmd!
    if defined showOutput echo -------------------------------------------------------------------------------
    echo !time! - proc!nextProc!: starting !cmd!
    2>nul del %lock%!nextProc!
    %= Redirect the lock handle to the lock file. The CMD process will     =%
    %= maintain an exclusive lock on the lock file until the process ends. =%
    start /b "" cmd /c %lockHandle%^>"%lock%!nextProc!" 2^>^&1 !cpu%%N! !cmd!
  )
  set "launch="

:wait
:: Wait for procs to finish in a loop
:: If still launching then return as soon as a proc ends
:: else wait for all procs to finish
  :: redirect stderr to null to suppress any error message if redirection
  :: within the loop fails.
  for /l %%N in (1 1 %startCount%) do (
    %= Redirect an unused file handle to the lock file. If the process is    =%
    %= still running then redirection will fail and the IF body will not run =%
    if not defined endProc%%N if exist "%lock%%%N" (
      %= Made it inside the IF body so the process must have finished =%
      if defined showOutput echo ===============================================================================
      echo !time! - proc%%N: finished !cmd%%N!
      if defined showOutput type "%lock%%%N"
      if defined launch (
        set nextProc=%%N
        exit /b
      )
      set /a "endCount+=1, endProc%%N=1"
    ) 9>>"%lock%%%N"
  ) 2>nul
  if %endCount% lss %startCount% (
    1>nul 2>nul ping /n 2 ::1
    goto :wait
  )

2>nul del %lock%*
if defined showOutput echo ===============================================================================
echo Thats all folks!

Answer 1

The digit before a redirection symbol is the stream number to redirect.
The default stream is 1, when no number is present, so 1>... and >... are equivalent.

stream 1 is the standard input/output stream, 2 is the standard error stream.

A command can output to multiple streams and it's allowed to redirect each of them to a different destination.

So 2>nul and 1>nul simply said that the error output and the normal output will be redirected to nul. So nothing will be outputted.