Monetisation is nowadays growing from research to standardisation. Monetary valuation has been used in policy making for decades mostly in cost-benefit analyses, since the early 30s in the United States. More recently in the European Union where this approach is observed since the 90s, in directives such as the National Emission Ceilings Directive (NEC directive (2001/81/EC), EU research projects such as the National Energy Education Development project (NEED (2007)), and many others. By the way, an umbrella of different concepts stays under the monetization term such as: market price, abatement costs, societal costs & benefits etc.
Mr. Philipp Preiss clarify the definition as follow, based on ExternE (2005) [1, 2] researches:
Externalities arise, when the social or economic activities of a participant have negative or positive impacts on another participant and these impacts are not fully accounted for or compensated by the first participant.
External costs are externalities that are transformed into monetary values. They are the share of damage costs which is not internalised.
Monetisation is promoted for several reasons. In the first hand, with monetisation of externalities the monetary values are shown explicitly. However, without monetisation the impacts are implicitly monetised by the difference of the internal costs of corresponding alternatives (i.e. a decision between alternative technologies or policies). Or in other words, if impacts are not monetised the external costs are taken into account as if they were zero.
On the other hand, monetisation offers an additional indicator but must not be the only basis for decision-making. At member state level, environmental agencies are involved, such as the German Federal Environmental Agency (UBA):  or the French ministry of environment (MEEM) who held a specific workshop on monetisation of goods and environmental services through life cycle assessment in 2017.
Currently two ISO documents (ISO 14007 and ISO 14008) are under construction in order to provide guidance and allow a wider use of monetisation.
Mr. Preiss points out some limits regarding the comparability of results of various studies. Studies are different regarding their comprehensiveness. There is a lack of transparency if the underlying assumptions and models are not displayed. And generally, there is the lack of a widespread state of the art approach.
Mr. Chanoine (Deloitte) proposed a practical guidance issued from a study carried out with the industrial consortium SCORELCA in 2012  and followed by a second study also driven by SCORELCA in 2016 of which the results are not published yet. It is based on operational recommendations on how to use monetary valuation to monetize LCA results. The proposal is an iterative approach with a go/no go process in two steps. First, a set of questions allows to determine the necessity to carry out monetisation, and how it should be done. Then nine information sheets provide explicit implementation guidance.
As a first conclusion, monetary valuation in LCA should be used when identified as necessary, depending on the objectives of the study. It has been presented as an approach to solve trade-offs. As a counterpart monetisation remains an approach potentially complex to implement due to the set of parameters to be considered in correlation to set monetary values, moreover considering the potential subjectivity of those parameters. Also, it remains especially complex when it is required to develop specific monetarisation factors, which may request time, funding, some specific competences investments and political decisions.