I think it is useful to think of four sources of descriptive metadata in libraries.

我觉得把图书馆中的描述性元数据分为四类对于我们不无裨益。

youdao

The paper analyzes four sources of the bullwhip effect: demand signal processing, rational game, order batching, and price variations.

对牛鞭效应的成因，从需求信号处理、订单批量、价格波动、短缺博弈四个方面进行了分析。

youdao

Songjiang County, Lishui City, four sources to the Dragon Hall East, west Branch (pit), west Village in the west side of the tributaries, so named.

丽水市松阳县四都源至龙殿分东、西二支流(坑)，西坑村在西边的支流边上，故以此命名。

youdao

Some mountainous regions benefit from hydropower. Less than four percent of the nation's electricity demand is met with renewable sources like solar and wind power.

VOA: standard.2010.03.23

Matthews says that that 97% of seniors with a prescription drug benefit get it from one of four sources: a former employer, Medicaid, Medicare-funded HMOs that are already in existence or from Medigap, a supplemental insurance policy seniors buy with their own money.

FORBES: Bush's Medicare Fix

Despite this, it has already set aside money from sales to fund water projects, including one in a village of 10, 000 people in the Tamil Nadu region of India, where Belu has created four sources of water for the town, sanitation blocks for children and hygiene education.

CNN: Eco-friendly bottle for ethical water

The demand for water originates from four main sources, namely, agriculture, production of energy, industrial uses and human consumption.

UNESCO: World Water Assessment Programme (WWAP)