Let $\left(\Omega,\mathcal{A},P\right)$ be a probability space and $(\mathcal{F}_t)_{t\in\{0,\ldots,T\}}$ be a filtration.
Definition 1: Let $Q_1$ and $Q_2$ be two equivalent measures and $\tau:\Omega\rightarrow\{0,\ldots,T\}$ a stopping time. Then the pasting of $Q_1$ and $Q_2$ in $\tau$ is the measure $\tilde Q$ defined by $$\tilde Q(A)=E_{Q_1}[Q_2(A|\mathcal{F}_\tau)]$$ for $A\in\mathcal{F}_T$.
I am reading trough some notes that use the measure $\tilde Q$ defined above but I don't know the meaning of the expression $Q_2(A|\mathcal{F}_\tau)$. My guess is: $ \ Q_2(A|\mathcal{F}_\tau)= E(1_{A}|F_\tau)$.
Any clarifications on this would be great
Usually, when $\mathbb P$ is a probability measure and $\mathcal F$ a $\sigma$-algebra, $\mathbb P\left(A\mid \mathcal F\right)$ refers to $\mathbb E_{\mathbb P}\left[\mathbf 1_A\mid\mathcal F\right]$ (that is, the conditional expectation is taken with respect to the measure $\mathbb P$).
The formula you wrote $\ Q_2(A|\mathcal{F}_\tau)= E(1_{A}|F_\tau)$ is not wrong, but since there are different probability measures involved, it is better to write $\ Q_2(A|\mathcal{F}_\tau)= E_{Q_2}(1_{A}|F_\tau)$ in order to avoid ambiguities.