1. In `set /a S=%t%%%86400%%3600%%60 2>nul`, `%t%` is a variable. The `%%` is used to escape the percent sign in batch processing, so that the percent sign is treated as a literal character instead of a variable delimiter.
2. In `ping -n 2 127.0|set/p l= 已计时间: %H%:%M%:%S%`, `set/p l=` is used to read input from the pipe. It reads the output from the `ping` command and assigns it to the variable `l` while displaying the specified prompt "已计时间: %H%:%M%:%S%".
3. `set /a t+=1` is an increment operation in batch processing. It is equivalent to `set /a t=t+1`, which increments the value of the variable `t` by 1.