Rainwater is considered an alternative water source because it is a sustainable source of water that is not supplied from fresh surface water or groundwater. Rainwater can be harvested from roofs or other surfaces and stored for later use. Rainwater can offset the demand for freshwater and reduce the pressure on water resources. Rainwater can be used for various purposes, such as irrigation, toilet flushing, cooling, and cleaning. LEED projects can earn credits for using alternative water sources under the Water Efficiency category. References: Best Management Practice #14: Alternative Water Sources1; LEED v4 BD+C Reference Guide, Water Efficiency, page 51

ASHRAE 62.1 is the standard for ventilation and acceptable indoor air quality in commercial and institutional buildings. It specifies the minimum ventilation rates and other measures to provide good indoor air quality for occupants. LEED v4 references ASHRAE 62.1 as a prerequisite for the Indoor Environmental Quality credit category.
ASHRAE 62.1 from the LEED v4 Green Associate Candidate Handbook, page 18.
Enhance Indoor Environmental Quality (IEQ) from the WBDG website, section on Adequate Ventilation and Exhaust.

One of the requirements for the Sustainable Sites Credit: Rainwater Management is that projects must use Green Infrastructure (GI) and Low Impact Development (LID) techniques within the LEED project boundary.
GI and LID are design approaches that mimic natural hydrologic processes to manage stormwater runoff at its source. GI and LID techniques include strategies such as rain gardens, bioswales, permeable pavements, green roofs, rain barrels, cisterns, and vegetated buffers. The other options are not requirements for the Sustainable Sites Credit: Rainwater Management. Maximizing open space within the LEED project boundary is a requirement for the Sustainable Sites Credit: Open Space. Calculating the total rainwater runoff for 75th percentile rainfall events is a requirement for the Sustainable Sites Credit: Site Assessment. Using a combination of both structural and non-structural measures to manage a 90th percentile rainfall event is an option for meeting Option 1 of the Sustainable Sites Credit: Rainwater Management. References: LEED Green Associate Candidate Handbook, page 27; USGBC, [Sustainable Sites], page 4-5.

Building commissioning is a quality assurance process that begins during the design phase and continues through construction, occupancy, and operation of the building. It ensures that the building's systems are designed efficiently, installed correctly, and operate as intended. The commissioning process involves a series of inspections, tests, and adjustments of building systems, followed by a review of the system design and operation with the building's operations team. References: LEED Green Associate Candidate Handbook, U.S.
Green Building Council resources

Reused materials are materials that have been used before and are incorporated into a new building without significant alteration1. They are different from recycled materials, which are materials that have been reprocessed into new products or materials2. Reused materials reduce the demand for new resources and the amount of waste sent to landfills1. LEED v4 encourages the use of reused materials by awarding points for Building Product Disclosure and Optimization - Sourcing of Raw Materials3.
Construction Waste Management | WBDG - Whole Building Design Guide1
Why plastic waste is an ideal building material - BBC Future2
LEED v4 | HPAC Engineering3