Attribution answers the question of how critical you were in making that specific investment that happen at the fund. For example - did you source the deal from external channels or were given the deal internally due to your particular domain expertise, for instance? The point is LPs need to believe that once you left the old fund, your personal network is strong enough to attract the best founders to your new fund. To do this, they are likely to require the following in diligence:

1) require the permission of the partners/organization to share the performance of the deals you worked with LPs 

2) allow LPs to diligence records of deal related performance data and information such as Deal Memos

3) diligence records showing that you sourced and invested in those deals