A process instance can have several variables.
To get process instance variables:
GET api/enterprise/process-instances/{processInstanceId}/variables
Where, processInstanceId is the Id of the process instance.
To create process instance variables:
POST api/enterprise/process-instances/{processInstanceId}/variables
          To update existing variables in a process instance:
PUT api/enterprise/process-instances/{processInstanceId}/variables
          Example response:
{
     "name":"myVariable",
     "type":"string",
     "value":"myValue"
}
Where:
- 
name - Name of the variable
 - 
type - Type of variable, such as string
 - 
value - Value of the variable
 
To update a single variable in a process instance:
PUT api/enterprise/process-instances/{processInstanceId}/variables/{variableName}
To get a single variable in a process instance:
GET api/enterprise/process-instances/{processInstanceId}/variables/{variableName}
To get all process instance variables:
GET api/enterprise/process-instances/{processInstanceId}/variables
          To get a specific process instance variable:
GET api/enterprise/process-instances/{processInstanceId}/variables/{variableName}
          To delete a specific process instance variable:
DELETE api/enterprise/process-instances/{processInstanceId}/variables/{variableName}