What is call appearance and line appearance in IP phones?

[Detlef Lechner]

Hello,

Synaptic: "Twinkle is a voice over Internet Protocol (VoIP) SIP Phone: A 
softphone for making telephone calls using SIP over an IP network. 
Twinkle supports direct IP phone to IP phone communication or a network 
using a SIP proxy to route your calls. Twinkle provides making basic 
voice calls, and in addition the following features (regardless of the 
services that your VoIP service provider might offer): two call 
appearances (lines), ..."
What is a »call appearance«?  I have found a description but I still 
don't understand what a call appearance is: 
http://www.cisco.com/en/US/products/hw/routers/ps331/products_qanda_item09186a0080093c4c.shtml: 
"What are call appearances? A directory number can be associated with a 
call appearance or, more briefly, a call. A call appearance is a 
connection between two or more directory numbers, for example, when one 
telephone user places a call to another. Directory numbers can carry 
several call appearances concurrently, such as when a telephone has a 
call waiting feature or a conference feature. - At any time, each call 
appearance is in one particular state. For example, a call appearance 
might be in an alerting state (ringing or flashing), a held state (on 
hold), or an active state, which means voice or data can flow end to 
end. The Telephone Manager provides a large number of functions that you 
can use in order to initiate, answer, or otherwise manipulate call 
appearances."
What is a »line appearance«?  The following text seems to imply that it 
is programmable button: Book (Google) Allan Sulkin: »PBX Systems for IP 
Telephony«, McGraw-Hill p 319: "Standard telephones support a single 
line appearance. Digital telephone models are available that support  a 
single line appearance or multiple line appearances. Each programmed 
line appearance is a distinct PBX system directory number. Some PBX 
systems support multiple call appearances (multiple programmable buttons 
supporting the same directory number) instead of distinct line numbers.  
--  Programmable buttons not used for line/call appearance usually are 
used for one-button feature access or speed dialling. Some single line 
analog telephones may be equipped with a limited number of programmable 
line/call appearance buttons but have additional feature/speed dialling 
programmable buttons. Programmable line/feature buttons usually have 
associated LED or LCD indicators to show the status of a line appearance 
or frature."
Both terms seem to have been standardized in a standard which I have no 
access to.

A happy New Year!
Detlef Lechner