Confused about BER (Basic Encoding Rules)

Confused about BER (Basic Encoding Rules)

I'm trying to study and understand BER (Basic Encoding Rules).
I've been using the website http://asn1-playground.oss.com/ to experiment
with different ASN.1 objects and encoding them using BER.
However, even the simplest encodings seem to confuse me.
Let's take a simple ASN.1 schema:
World-Schema DEFINITIONS AUTOMATIC TAGS ::=
BEGIN
Human ::= SEQUENCE {
name UTF8String
}
END
So basically this is just a SEQUENCE with a single UTF8String type field
called name.
An example of a value that matches this sequence would be something like:
{ "Bob" }
So, using http://asn1-playground.oss.com/, I produce the BER encoding of
the following data:
some-guy Human ::=
{
name "Bob"
}
I would expect this to produce one sequence object, followed by a single
string object.
What I get is:
30 05 80 03 42 6F 62
Now, I understand some of this encoding. The first octet, 30, is the
identifier which tells us that a SEQUENCE type is the first object. The 30
is 00110000 in binary, which means that we have a class of 0, a PC
(primitive/constructed) bit of 1 (meaning constructed), and a tag number
of 10000 (16 in decimal) which means SEQUENCE
So far so good. The next value is the LENGTH in bytes of the SEQUENCE,
which is 05.
Okay, still so far so good.
But then... I'm totally confused by the next octet 80. What does that
mean??? I would have expected a value of 00001100 (for tag number 12,
meaning UTF8String.)
The bytes following the 80 are pretty straightforward: the 03 means Length
of 3, and the 42 6F 62 is just the UTF8String value itself, "Bob"