The trailing @ holds a line of data for subsequent INPUT statements, but releases that line of data when SAS returns to the top of the DATA step to begin building the next observation. The double trailing @ holds a line of data for subsequent INPUT statements even when SAS starts building a new observation. In both cases, the line of data is released if SAS reaches a subsequent INPUT statement that does not contain a line-hold specifier.

这段话什么意思啊，@和@@的区别这里看不懂，求助大牛帮忙中文解释下，多谢多谢