  The widespread adoption of third-party libraries (TPLs) in software
development has accelerated the creation of modern software. However, this
convenience comes with potential legal risks. Developers may inadvertently
violate the licenses of TPLs, leading to legal issues. While existing studies
have explored software licenses and potential incompatibilities, these studies
often focus on a limited set of licenses or rely on low-quality license data,
which may affect their conclusions. To address this gap, there is a need for a
high-quality license dataset that encompasses a broad range of mainstream
licenses to help developers navigate the complex landscape of software
licenses, avoid potential legal pitfalls, and guide solutions for managing
license compliance and compatibility in software development. To this end, we
conduct the first work to understand the mainstream software licenses based on
term granularity and obtain a high-quality dataset of 453 SPDX licenses with
well-labeled terms and conflicts. Specifically, we first conduct a differential
analysis of the mainstream platforms to understand the terms and attitudes of
each license. Next, we propose a standardized set of license terms to capture
and label existing mainstream licenses with high quality. Moreover, we include
copyleft conflicts and conclude the three major types of license conflicts
among the 453 SPDX licenses. Based on these, we carry out two empirical studies
to reveal the concerns and threats from the perspectives of both licensors and
licensees. One study provides an in-depth analysis of the similarities,
differences, and conflicts among SPDX licenses, revisits the usage and
conflicts of licenses in the NPM ecosystem, and draws conclusions that differ
from previous work. Our studies reveal some insightful findings and disclose
relevant analytical data, which set the stage for further research.
