### Human: Please give me the types of this contract: This <mask> agreement is in regards to: (i) that certain Master Product Purchase
Agreement, by and between Smart Sand, Inc. (“Smart Sand”) and Liberty Oilfield
Services, LLC (“Liberty”), dated March 8, 2017, as amended by that certain First
Amendment to Master Product Purchase Agreement, dated effective as of May 1,
2017, and those certain letter agreements entered into by the parties from time
to time (as amended, the “PPA”); (ii) that certain Railcar Usage Agreement,
dated March 8, 2017, as amended by that certain First Amendment to Railcar Usage
Agreement, dated effective as of May 1, 2017, and those certain letter
agreements entered into by the parties from time to time (as amended, the
“RUA”); and (iii) that certain Transportation and Transloading Services
Agreement, dated August 7, 2018, by and between Smart Sand, SSI Bakken I, LLC,
and Liberty, and those certain letter agreements entered into by the parties
from time to time (as amended, the “Transloading Agreement”). All capitalized
terms contained in this letter agreement and not otherwise defined shall have
the meanings ascribed to such terms in the PPA, RUA and Transloading Agreement,
as applicable.
Per our discussion, Smart Sand and Liberty each hereby agrees to the following:### Assistant: It seems like to be Letter.