为什么不需要为springboot项目中的某些依赖项指定版本?

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>1.5.6.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.dgs</groupId>
    <artifactId>n-tier-and-jackson</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>n-tier-and-jackson</name>
    <description>Demo project for Spring Boot</description>

    <properties>
        <java.version>1.8</java.version>
        <paypal.rest.easy.version>2.3.3-RELEASE</paypal.rest.easy.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-jersey</artifactId>
        </dependency>

        <dependency>
            <groupId>com.paypal.springboot</groupId>
            <artifactId>resteasy-spring-boot-starter</artifactId>
            <version>${paypal.rest.easy.version}</version>
        </dependency>

...

例如,在这段代码中,您不需要指定springbootstarter的版本,但需要指定resteasyspringbootstarter的版本。SpringBootStarter球衣的版本来自哪里?谢谢您!