Register

The term register is widely used in sociolinguistics to refer to varieties according to use.  One and the same person may use different linguistic items to express more or less the same meaning on different occassions. E.g. in writing a letter a person might start. "I am writing to inform you that...", or "I just wanted to let you know that...". In the I case it is more official while the II shows the solidarity with the person you are writing to. Haliday distinguishes 3 general types of dimensions: 

1. field,

2. mode,

3. tenor.

Field is concerned with the purpose and subject matter of communication, in other words, it refers to why and about what the communication takes place. 

Mode refers to the means by which communication takes place, i.e. by speech or writing. 

Tenor depends on the relations between participants. The speaker defines his relationship with the person he is communicating with.

Hymes uses different terms: social distance (solidarity), status or power, formality (the piece of writing is formal or informal language), function (referential and affective function). When we want to give information it is referential and when we express our emotion it is affective.