Web of Science的记录格式最近增加了几个新字段，包括U1和U2.

详见：http://images.webofknowledge.com/WOKRS519B3/help/WOK/hp_usage_score.html

Last 180 days. This is the count of the number of times the full text of a record has been accessed or a record has been saved in the last 180 days. This count can move up or down as the end date of the fixed period advances.

Since 2013. This is the count of the number of times the full text of a record has been accessed or a record has been saved since February 1, 2013. This count can increase or remain static over time.

U1: 近180天内，全文的访问次数，或保存该记录的次数。

U2: 2013年２月１日至今的全文的访问次数，或保存该记录的次数。

CiteSpace 4.0.R2可以在共被引网络中以白底红圈显示这两组数值。

详见：https://sites.google.com/site/citespace101/first-example/4-0/4-0-web-of-science　

中文解说见http://blog.sciencenet.cn/blog-496649-838067.html 第16条。