subframe value in clipBegin/clipEnd

I'm stepping into precision time on SMIL.

Could someone give me some explanation and examples of "subframe" in
plainer language ?

I see there is a definition of clipBegin/clipEnd attributes in 7.5.1
MediaClipping Attributes of SMIL 2.0.  But I do not understand the
point of the definition of "subframe" below:

 Subframes         ::= Digit Digit; smpte range = 00-01, smpte-30-drop range = 00-01, smpte-25 range = 00-01

and 

 The time value has the format hours:minutes:seconds:frames.subframes.
 If the subframe value is zero, it may be omitted. Subframes are
 measured in one-hundredths of a frame.

How can I measure subframes only by 00 and 01 one-hundredth of a frame ?
Or is that simply an error ?

And one more. What was the sentence below in the same section supposed
to do ? What "level 1" means ?

 Note: additional BNF for level 1 extensions defined later

Thanks in advance.

Yoshihisa Gonno <ygonno@cocoa.freemail.ne.jp>