Expense models and expense categories are typically used to associate expenses with ledger accounts. This process is explained in the Accounting section of our online documentation.
However, you can also use expense categories to categorize expenses in other ways. For example, one of our current customers performs grounds keeping at a number of different sites. For each site, they create a single work order each week (since this is how their clients prefer to account for the work). During the week, workers might do jobs like "Mowing", "Pruning", "Garden Maintenance", and so on. At the end of the week, the workers submit time-cards saying how much time they spent on each possible type of job.
To account for this, the ground skeeping company creates an expense category for each type of job. They then use the time-card information to record labor expenses on each site's single work order. They give each labor line item an expense category that indicates the type of work done.
This approach lets you categorize different types of work on a single work order. You couldn't use normal work categories for this purpose; since a work order can only have one work category, you'd have to make separate work orders for each work category. By using expense categories, you can distinguish between multiple types of work on a single work order.
An alternative approach to this situation would be to create a number of separate "Inside Labor" records, with different records for each worker and each activity. For example, you might have "Inside Labor" records for "Joe Mowing", "Joe Pruning", "Joe Garden Maintenance", and so on. This may be somewhat more complicated than using expense categories, but it has the advantage of working well with the Web Access module: workers can use the web site to fill in their hours directly, rather than going through time-cards.