History lines are basically a diary. Use history lines to read what happened to an item and add comments that can be helpful for handling the item. There are two types of history lines, system generated lines and user created lines.
System generated history lines are created by the system. For example, when an item is changed a history line could be generated revealing when the item was changed, who applied the change, and what the change was. System generated lines cannot be changed by users. However, users can add extra information to system lines. Not everything that happens to an item is registered in the history lines. What is registered and what is not, is set by the System Administrator in the system settings.
User history lines are an excellent means to pass on information that can be helpful when handling the item. User history lines can also record questions, contain copies of e-mail, hold a summary of a telephone call or reference other material that is used to handle the item. A user created history line also shows when the line is created and who created the it. Users can modify the subject and the information of the user created history line.
You can enter the amount of time you spend working on an item by using the spent time fields to record the time in hours and minutes. This information can be added up for project planning or to track the amount of hours to bill your customers, for example.