<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
    <item>
        <shape android:shape="rectangle">
            <stroke android:width="1dp" android:color="#BE55DA"/>
            <solid android:color="#00000000" ></solid>
        </shape>
    </item>

    <item android:top="1dp" android:bottom="1dp" android:left="1dp" android:right="1dp">
        <shape android:shape="rectangle">
            <stroke android:width="1dp" android:color="#BE55DA"/>
            <solid android:color="#00000000" />
        </shape>
    </item>
</layer-list>

위 item과 아래 item의 차이는 무엇인가요? 그리고 왜 이 둘을 layer-list로 중첩시키는건가요?