What is the match rule of Feature Dependencies? - eclipse

Question

When I set the required features of a feature, I add "test.feature.A" and don't specify its version, why there is an Error icon on this feature? I don't see any error message.
screenshot
test.feature.A :
<?xml version="1.0" encoding="UTF-8"?>
<feature id="test.feature.A" label="A" version="1.0.0.qualifier">
<description url="http://www.example.com/description">
[Enter Feature Description here.]
</description>
<copyright url="http://www.example.com/copyright">
[Enter Copyright Description here.]
</copyright>
<license url="http://www.example.com/license">
[Enter License Description here.]
</license>
</feature>
test.feature.B
<?xml version="1.0" encoding="UTF-8"?>
<feature id="test.feature.B" label="B" version="2.0.0.qualifier">
<description url="http://www.example.com/description">
[Enter Feature Description here.]
</description>
<copyright url="http://www.example.com/copyright">
[Enter Copyright Description here.]
</copyright>
<license url="http://www.example.com/license">
[Enter License Description here.]
</license>
<requires>
<import feature="test.feature.A"/>
</requires>
</feature>
Moreover, if I specify its version and don't specify its Match Rule, what is the actual match rule ?
<requires>
<import feature="test.feature.A" version="1.0.0.qualifier"/>
</requires>