Occasionally, peering requirements include a “Not to Exceed” traffic ratio which specifies that aggregate peering traffic ratios will not exceed a particular ratio. This ratio is quoted in the "Out:In" form, as in, "traffic ratios must not to exceed 2:1.”
The more interesting question is, "Are Peering Ratios a rational discriminator or is it just a rationale for saying 'No'?"
We did research with the peering coordinators and determined that there are a handful of arguments that are given for not peering with those that don't have a balanced ratio. These peering ratio arguments are refuted in "The Folly of Peering Ratios." In a nutshell, peering traffic ratios are a mechanism for ISPs to deny peering with others that do not look like them. Ultimately, ISPs with peering ratio requirements do so as a way to tease away potential customers out of the mix of potential peers.