update history

docs/about/history.md

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 History
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-Tax-Calculator will be split into an income tax model and a payroll tax model, as Taxcalc-Income (the original Tax-Calculator) and Taxcalc-Payroll. Taxcalc-Income will be functioning merely as an income tax calculator. Taxcalc-Payroll is planned to be developed into a larger model, hosting all of the payroll tax related functions which includes tax offset.
+## Functions of Taxcalc-Payroll
 
+Taxcalc-Payroll is a payroll tax model, consisted of (1) payroll tax functions  (2) employer side payroll tax offset function 
 
 ## Origins of Taxcalc-Payroll
 
-Taxcalc-Payroll is a payroll tax model, consisted of (1) payroll tax calculator, as descendant of [Tax-Calculator](https://github.com/PSLmodels/Tax-Calculator).  (2) payroll tax offset function 
+Taxcalc-Payroll orginiats from [Tax-Calculator](https://github.com/PSLmodels/Tax-Calculator). 
+
+In the past, Tax-Calculator was the only microsimulation model for both income tax analysis and payroll tax analysis. 
+
+However, in the progess of model expansion, many payroll tax featured developments are newly made (for example, employer side payroll tax offset function). These developments are beyond the realm of Tax-Calculator. It has become necessary to build Taxcalc-Payroll as a new platform to host them. 
+
+From the perspective of model management, many research organizations also build their tax model systems by an income tax model and payroll tax model. It is thus considered better to have two models, Tax-Calculator and Taxcalc-Payroll serveing as income tax calculator and payroll tax calculator.
+
+Taxcalc-Payroll inherits the software infrastructure from Tax-Calculator. These two models are highly integrated, with most tax calaulation functions can be called by each other. Users can still conduct both income and payroll tax analysis in any one of the models.
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+