These tags refer to the types of resources offered. For instance, if science lessons plans for Kindergarten through third grades are offered, then the tags might be: science, lesson plans, K-3 education, primary education.
Does this mean that every specific resource should be tagged?
No. In an attempt to capture the site holistically, we try to include only those tags that pertain to most of the resources offered. For example, say a site offers lesson plans for 8th, 10th, and 12th grades in the subject areas of biology and chemistry. Instead of tagging all of these separately, we can capture all six aspects in three tags: lesson plans, secondary education, science.