This should be a 6 column (or 7 with additional k=v pairs) tabular file of link data.

Controls which end(s) of the link are processed for each bin. The start of the link is considered to be the end defined by the first link line, and the link end is defined by the second link line. For example, if -link_end=start then a bin will contain the sum (or number if -num is used) of links that start at that bin. If -link_end==end then links that end at that bin will be used. Finally, -link_end=both will make the bin value reflect the total outgoing and incoming links. (from the Circos Manual)

Controls which end(s) of the link are processed for each bin. The start of the link is considered to be the end defined by the first link line, and the link end is defined by the second link line. For example, if -link_end=start then a bin will contain the sum (or number if -num is used) of links that start at that bin. If -link_end==end then links that end at that bin will be used. Finally, -link_end=both will make the bin value reflect the total outgoing and incoming links. (from the Circos Manual)

If the end of a link spans bins, each bin will contain a +1 contribution from the link. In other words, the sum of bin values may be larger than the number of links. (from the Circos Manual)

This only works when you select 'stacked histograms' in the 'Output Style' section.