Callsign
A callsign is a personal identifier based on internationally recognized country codes. They consist of country code prefix, a number (usually 0 to 9) which may identify a region within that country and a 1, 2, or 3 unique personal identifying suffix. This suffix may be followed by a further suffix, such as P (portable), M (mobile), or MM (maritime mobile). The P, M and MM suffixes are not used in the USA.
For example VK4XXX identifies the operator as an Australian (VK) from Queensland (4) with personal identifier (XXX). If this operator was traveling in Tasmania (VK7), a voice call identifier would be "VK4XXX P7" or "VK4XXX portable 7".
Increasingly, reciprocal arrangements between countries are negotiated that allow traveling operators to use their home callsign with a suffix to identify their traveling status. These reciprocal arrangements usually have a time limit (often 90 days), and remove the necessity for traveling operators to apply for a temporary license to operate in "foreign" countries.
Many countries have slight variations to these general principles to identify the various classes of operator available.