California Code, Government Code - GOV § 66498.9

(a) To establish a procedure for the approval of tentative maps that will provide certain statutorily vested rights to a subdivider.

(b) To ensure that local requirements governing the development of a proposed subdivision are established in accordance with Section 66498.1 when a local agency approves or conditionally approves a vesting tentative map.  The private sector should be able to rely upon an approved vesting tentative map prior to expending resources and incurring liabilities without the risk of having the project frustrated by subsequent action by the approving local agency, provided the time periods established by this article have not elapsed.

(c) To ensure that local agencies have maximum discretion, consistent with Section 66498.1, in the imposition of conditions on any approvals occurring subsequent to the approval or conditional approval of the vesting tentative map, so long as that discretion is not exercised in a manner which precludes a subdivider from proceeding with the proposed subdivision.