### Tonnage Tax System: Overview and National Implementations

The tonnage tax system is a specialized tax regime applied to shipping companies based primarily on the net tonnage (i.e., the size or capacity) of their fleet, rather than on the traditional measure of corporate profit. This tax model is designed to simplify the fiscal environment, reduce administrative burdens, and encourage growth in the maritime sector. Different nations adopt varying approaches to the implementation and duration of tonnage taxes, reflecting their unique economic priorities, maritime heritage, and policy goals. Below, we delve into the nuances that characterize the two differing regulatory approaches as encapsulated in the given answers: the permanent incorporation into general tax law, and the adoption as a temporary measure under special laws.

#### Permanent Feature of the General Tax Law
In some countries, the tonnage tax system is a permanent feature of the general tax regime. This integration signals a long-term commitment to providing a stable and predictable fiscal environment for maritime enterprises. The primary rationale for making the tonnage tax a lasting part of the tax landscape is to bolster the international competitiveness of the nation’s fleet, attract investment in the shipping sector, and secure maritime jobs.

**Legislative Stability and Economic Rationale**
Making the tonnage tax system a fixed element of the general tax law provides shipping companies with a long-term perspective regarding their financial planning and investment strategies. This stability is crucial for capital-intensive industries, where investments are substantial and payback periods are lengthy. Governments opting for this approach often view the maritime sector as vital to their economic infrastructure, warranting such enduring support. Additionally, a permanent regime is less likely to be affected by political changes, ensuring consistency in policy enforcement.

**Impact on Domestic Shipping Industry**
A permanent tonnage tax regime can lead to sustained growth in the domestic fleet. It offers incentives for both national and foreign investors to register ships under the country’s flag, thereby increasing the gross tonnage and potentially enhancing the global standing and influence of the nation in maritime affairs.

#### Governed by a Special Law and Temporary
Conversely, other nations might implement the tonnage tax system as a temporary measure, often enacted through special legislation that includes a sunset provision. This temporary nature typically reflects a trial approach to economic incentives, allowing governments to test the system’s efficacy and adjust it as needed based on evaluative outcomes.

**Flexibility and Policy Adaptation**
The temporary status of a tonnage tax system under special law allows a government to retain flexibility in policy making. This can be particularly advantageous in responding to international economic pressures or changes in maritime trade dynamics. Sunset provisions enable regular reassessment and fine-tuning of the tax system, ensuring that it meets the current economic objectives without committing to long-term impacts that might prove disadvantageous.

**Evaluation and Adjustment**
By making the tonnage tax system temporary, governments can set predefined periods after which the tax’s performance is reviewed. This review process can lead to enhancements in the system, such as adjusting tax rates, modifying eligibility criteria, or even a comprehensive overhaul of the scheme based on empirical evidence and sector feedback.

### Conclusion
The decision to adopt the tonnage tax system as either a permanent part of general tax law or as a temporary measure under special legislation is indicative of a country's broader fiscal policies and economic strategies. Where permanence provides stability and long-term growth prospects for the maritime sector, temporariness allows for agility and adaptiveness in rapidly changing global economic landscapes. Both approaches aim to enhance the competitiveness and viability of the national fleet, albeit through fundamentally different avenues of policy design and implementation. The choice between these paths depends significantly on national priorities, economic conditions, and the strategic importance of the shipping industry within a specific national context.