Think of ISO's commercial lines program as building blocks. Every policy has the same base, and the insurer snaps on one block for each line of insurance the business needs. Adding or removing a line means adding or removing a block, not writing a new policy.
The base:
Common Policy Declarations: the named insured, mailing address, policy period, which coverage parts are included and the premium for each.
Common Policy Conditions: conditions that apply to every coverage part, such as cancellation and premiums.
Each block is a coverage part, and each coverage part has its own declarations, one or more coverage forms, and any endorsements. Property needs one more piece, a causes of loss form, because the property coverage form does not by itself say which perils are insured. A liability coverage part responds to legal liability, so it has no causes of loss form.
Common modules include commercial property, commercial general liability, commercial crime, commercial inland marine, equipment breakdown, commercial auto and farm. Example: a bakery with property and general liability adds crime midterm. Crime comes in as one more coverage part under the existing common declarations and conditions; no second set of common forms is needed.