31 C.F.R. § 203.3
A financial institution that participates in PATAX and/or the investment program must be a TT&L depositary. There are three kinds of TT&L depositaries. A collector depositary is a TT&L depositary that accepts paper Federal tax payments and also may accept electronic Federal tax payments, but does not accept direct investments or SDIs. A retainer depositary is a TT&L depositary that accepts electronic and/or paper Federal tax payments and retains a portion ofthe tax deposits in its TIP main account balance. An investor depositary is a TT&L depositary that accepts direct investments, SDIs, or dynamic investments and may accept electronic and/or paper Federal tax payments and retain a portion of those tax deposits. Collector, retainer, and investor depositaries may accept term investments. Retainer and investor depositaries do not have to participate in PATAX.
31 C.F.R. §203.3