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Reference: Triparty-Repos by the NY FED, this describes the Repo Market, the terms and risk associated with it and the various calculations and formulae that govern the yield calculation, not just tri-party.
Opening Legleg: The value of the collateral provided is greater than the cash lent. This difference is called "the haircut". For example, if the value of the securities is $100 and the cash lent is $94, the haircut is $6.
Time between Opening and Closing legs is usually a day or two. This is the term of the repo. If during this the term of the repo is overnight, it is called an overnight; everything else is called a term repo. Term repos are more complicated due to margin calls. The observed price of the security rises or falls, there may be margin calls to preserve the haircut.
Closing leg:
Breaking News: In the last week of September of 2019 there was a huge problem in the repo market. ON Sept 23. Lenders lenders disappeared from the market, leaving many who wanted short term funding without takers; this created a a huge spike in the overnight and term rates. The commotion was quelled only when the NY FED stepped in providing liquidity in the market. Speculation abounds as to the root cause.